2. Week 2
Section 2.4, 2.5, 2.6 and section 2.7
Srabasti dutta

3. Equations with many variables
Sometimes an equation might have more than one variables.
Example: 2x + 3y = 4. As we learned in the other PPT files, x and y are variables.
The question might ask us to solve either for x or y.
Whenever you are asked to solve for a particular mean, you have to make that variable stand alone.
Example: Solve for x for 2x + 3y = 4. Here x is not standing alone. First of all it has 2 attached (multiplied) to it. Then 3y is added to it. So, first get rid of 3y: 2x + 3y – 3y = 4 – 3y  2x = 4 – 3y. Then, divide both sides by 2: 2x/2 = (4 – 3y)/2  x = (4 – 3y)/2 and that’s the solution for x (since it is now standing alone).
Solve for y: 2x + 3y – 2x = 4 – 2x  3y = 4 – 2x  3y/3 = (4 – 2x)/3  y = (4 – 2x)/3 and thus that’s the solution since y is now standing alone.

4. Functions
In the previous slide, we had x = (4 – 3y)/2 and y = (4 – 2x)/3.
So, we expressed x in terms of y in x = (4 – 3y)/2. Mathematically, we say that x is a function of y.
We expressed y in terms of x in y = (4 – 2x)/3. Mathematically, y is a function of x.
A function is a rule for determining uniquely values of one variable from values of one or more variable. When we say that x is a function of y, we mean that we can determine the values of x from the values of y. Thus, if y = 1, then we will have x = (4 – 3*1)/2 = (4 – 3)/2 = ½. If y = 2, then, x = (4 – 3*2)/2 = (4 – 6)/2 = -2/2 = -1.
In such situations, we say that y is the independent variable; x is the dependent variable (as its values depend on that of y’s).

5. Outlines
In sections 2.4 to 2.7, you will be learning about solving word problems.
Many of the word problems will involve concepts such as decimal numbers, rounding off numbers, area, perimeter, rates, discount, percentage, simple interest and other.
For many of such concepts, the book might not have included definitions.
So, the next few slides will explain many such concepts.

6. Decimal numbers We have learned about whole numbers and fractions.
Decimals are another kind of numbers.
Decimal numbers are denoted using points, known as decimal points, like 12.5, , , etc.
The first number after the decimal is known to be in tenth place; the second number is known to be in hundredth place; the third number in thousandth place and so on. Thus, 5 in 12.5 is in tenth place; 9 in is in hundredth place; 4 in is in thousandth place.
For a long decimal number, you might be asked to round it of to hundredth place or two decimal places. That means, there should be only two rounded-off numbers after the decimal place; if you are asked to round off to thousandth place or three decimal places, that means there should be three rounded off numbers after the decimal place.
Decimal Point

7. Decimal number place value
Here is the place-value for decimal numbers. Note that the place-values move from left to right (instead of whole number place-values which moved from right to left).
The place values start after the decimal point.
Example: 2.5 has one decimal place because it has just one number after the decimal point
Example: has three decimal places because it has three numbers after the decimal point.
Decimal Point (.)
1/10 = 0.1
1/100 = 0.01
1/1000 = 0.001
1/10,000 =
1/100,000 =
Places
Tenths
Hundredths
Thousandths
Ten-thousandths
Hundred-thousandths
First (one)
Second (two)
Third (three)
Fourth (four)
Fifth (five)

8. Converting decimal to fraction
Each decimal number can be converted into fraction. To do so, count the number of decimal places a number has and divide the number by 10, 100, 1,000, etc. according to the number of places
Example: 0.5 has one decimal place. Thus, 0.5 =
Example: 2.87 has two decimal places. Thus, 2.87 =
NOTE 1: When writing the number in fraction, we do not include the decimal point.
NOTE 2: The numbers before and after the decimal place are written together as one whole number in the numerator. The denominator has 10, 100, 1000 based on number of decimal places and the place-value table (refer to the previous slide).

9. Rounding off decimal numbers
Steps for rounding numbers
Locate the digit just to the right of the place you want to round off to
If that digit is less than 5, then omit it and all other remaining numbers to its. The digit to its left remain as it is.
If that digit is greater than 5 or equal to 5, then omit it and all other remaining numbers to its right, and add 1 to the digit to its left.
Example: Round off to two decimal places. 8 is in the second decimal place. The number to the right of 8 is 2. It is less than 5. Thus all the numbers from 2 to right is omitted. The final solution is 5.98.
Example: Round off to five decimal places. 1 is in the fifth decimal place. The number to its right is 7. It is greater than 5. Thus, 7 gets omitted and 1 (the number immediately to the left of 7) is increase by 1. The final solution is

10. Perimeter
Perimeter of a geometric figure is the sum of lengths of all its sides.
Example: Suppose we have a triangle with one side being of length 3 inches, the other side being of length 4 inches and the last side being of length 2 inches. Find the perimeter of the triangle.
Solution: add the lengths = = 9 inches.
Perimeter of a rectangle is 2L + 2W, where L and W represents length and width.
Perimeter of a square is 4L where L is the length of any one side.

11. Area Area is the total place (surface) enclosed within a figure.
Area of the above rectangle is the whole yellow-region.
Formulas: Area of a rectangle = LW where L is the length and W is the width
Area of a square = L2 where L is the length of one side of the square
Area of a triangle = , where b is the base of the triangle and h is the height.

12. rates Rate is a measurement of one quantity against another quantity.
It is written as a ratio
Example: A car travels 100 miles on 16 gallons of gas. Then mile to gas ratio is a rate: 100 miles to 16 gallons =
Or = 6.25 miles/gallon

13. proportions When two rates are equal, it is known as proportion.
If a/b = c/d are two equal ratios, then the statement
is known as a proportion.

14. Property of proportions
If , then ad = bc, that is product of numerator of first ratio and
denominator of second ratio is equal to the product of numerator of second ratio and denominator of first ratio.
Example:

15. Use of property of proportions
We use the property of proportions to solve simple equations.
Example 1 Solve for x:
Step 1: Apply the property: 4*x = 3*5  4x = 15 (multiplying both sides by 4 and then multiplying both sides by 5).
Step 2: Multiply both sides by reciprocal of 4:
Step 3: Write down the answer: x = 15/4 or or 3.75

16. Application of proportions
A woman drives her car 235 miles in 5 hours. At this rate, how
far will she travel in 7 hours?
Step 1: Convert the word-problem into a proportion:
235 miles to 5 hours = how far to 7 hours 
Step 2: Represent the unknown quantity with a variable x:
Step 3: Solve for x  235*7 = 5*x  1645 = 5x
Multiplying both sides by 1/5 (which is same as dividing both
sides by 5):  329 = x
Step 4: The woman will travel 329 miles in 7 hours

17. percent
During the recent election, you must have heard statements like “turnout in midterm elections is far lower, peaking at 48.7% .”
48.7% is read as 48 point seven percent
Percent means per hundred.
So, 48.7% means 48.7 out of 100

18. Converting percent to fractions
To convert percent into fractions, write the percent out of 100 and then reduce the fraction into lowest form
Example 1: 57% =
Example 2: 25% =

19. Converting percent to decimal
Step 1: Write the percent out of 100
Step 2: Reduce the fraction into lowest term
Step 3: Write the lowest-term fraction as decimal
Example: 30%
Step 1: 30% is 30 out of 100
Step 2:
Step 3: 3/10 = 0.3

20. Short-cut method for converting percent into decimals
To change percent to a decimal, drop the % symbol and move the decimal point two places to the left.
Example 1: 42% = .42 (note we have shifted two places to the left and put down the decimal point).
Example 2: 201% = 2.01
Example 3: 80.1% = .801
Example % = ._ _ 5(the decimal point was before 5. Shift it two places to the left; the two places are indicated by the blanks. In the blanks we put zeros. Thus, the final answer is 0.5% = 0.005
We have shifted two numbers to the left. The two numbers are 01. Thus
put a decimal point before 0 to get 2.01

21. Changing decimal to percent
There are two ways to convert a decimal to percent.
Method 1 (short-cut method): move the decimal point two places to the right and use the % symbol.
Method 2: Write the decimal as a fraction of lowest term and then multiply the top by 100; use the % symbol.

22. Example 1 Method 1: Convert 0.45 into percent
Move the decimal two places to the right and use the % symbol.
Thus, 0.45 = 45.% = 45%
Method 2: Write 0.45 as a fraction:
Multiply the top by 100:
Cancel the 100 from top and bottom and use the % symbol to get

23. Example 2 Convert 0.09 into percent
Step 1: Move the decimal point two places to the right:
.09  09.
Step 2: 09 is nothing but 9
Step 3: using % symbol we have 9% as the solution

24. Changing fractions to percent
To change a fraction into percentage, we can follow either of the two methods:
Method 1: Convert the fraction into a decimal and change the decimal into percent
Method 2: Multiply the fraction by 100 and simplify (that is make sure everything is in lowest term).

25. Example Convert ¾ into percent
Method 1: Step 1: ¾ = 0.75 in terms of decimal.
Step 2: Shift the decimal point two places to the right to get
75%
Method 2: Multiply ¾ by 100 and simplify:

26. Basic percent problems
The basic percent problems can be of three types:
What number is 20% of 52?
What percent of 52 is 27?
30 is 50% of what number?
To solve any of the above questions, you first have to convert
the word-problem into equation and then solve. We will only
look at number 1.

27. What number is 20% of 52? The number is unknown.
We know that the number = 20% of 52
Represent the number as n
Thus, n = 20% of 52. “Of “is nothing but multiplication.
Thus, we have the equation n = 20%*52
 n =

28. commission
Commission is the earning that a salesperson earn based on a percentage of amount the salesperson sell.
The commission rate is also always represented in percent.
Commission problems are nothing but percent application problem.
To find the commission, you convert the word problem into an equation and solve the equation.

29. Example -- commission
A real estate agent has a commission rate of 3%. If a piece of property sells for $94,000, what is her commission?
Commission = percent of earning.
Percent = 3%
Earning = $94,000
Therefore, commission = 3% of $94,000 =
Thus, the commission is $2,820

30. discount
Discounts are also percentage problems as discounts are always represented in percentage.
During a clearance sale, a three-piece suit that usually sells for $300 is marked 15% off. What is the discount? What’s the sales price?
Discount = 15% of $300 =
Sales price = Original price – discount = $300 - $45 = $255

31. Simple interest
Whenever we borrow money, we have to pay interest on the amount of money borrowed.
Whenever we deposit money in any account, the money earns interest.
There are two types of interest – simple interest and compound interest
In this course, we will only learn how to solve problems involving simple interest
A simple interest is nothing but the interest that is paid on the principal alone.

32. formula Principal – the amount that is invested.
Interest rate -- the percent of principal that is earned (in case of investment) or the percent of principal that needs to be paid (in case of borrowing money)
Interest – the amount of money earned
Therefore, the simple interest formula is I = PRT, where I = interest, P = principal, R = rate, T = time

33. Example
A farmer borrows $8,000 from his local bank at 7%. How much does he pay back to the bank at the end of the year when he pays off the loan?
Step 1: Write down the values of P, R and T: P = $8,000; R = 7%; T = 1
Step 2: Convert percent into decimal: 7% = 0.07
Step 3: Apply the formula I = PRT  I = $8,000*0.07*1 = $560
So, he pays back $8,000 + $560 = $8,560 at the end of the year.

34. Angles
Complimentary angles: if two angles add up to 90◦, we call them complimentary angles
Supplementary angles: if two angles add up to 180◦, we call them supplementary angles.
Examples:
Angle 43◦ and angle 47◦ are complimentary angles because = 90 degree
Angle 102◦ and 78◦ are supplementary angles because = 180 degree.
Find x such that x and 51◦ are (a). complimentary angles, (b). Supplementary angles.
(a). X + 51 = 90  x = 90 – 51 = 39 degree
(b). X + 51 = 180  x = 180 – 51 = 129 degree

35. Strategies for solving word problems
In summary, here are the rules you should follow in solving any word problems:
Read the problem and underline all the information that are given in the problem.
Write down what’s being asked to solve in the problem.
Whatever is asked being solved or whatever the unknowns are, represent it (them) with variables (x, y, z, w, etc).
Use formulas if applicable
Draw pictures to represent the problem. Often time it helps.
Write equations/algebraic expressions to represent the unknown variable in terms of the known variables/value/information given in the problem.