1. Equations
Mrs. Wallace
Math

2. Review Expressions
An expression has numbers and operation signs, but no equal sign. Expressions can include parentheses and different operations (+, -, x).
Sue had 12 flowers. Lily gave her 4 more flowers. Sue gave her teacher 3 of her flowers. How many does she have left?
12 flowers plus 4 flowers, minus 3 flowers
(12 + 4) – 3
16 – 3 13

3. Review Expressions with Variables
You can use a variable, or letter, to stand for any number that you do not know. Then write the expression.
8 + m
Find the value of 8 + (m – 2) if m = 6.
8 + (m – 2) Replace m with 6
8 + (6 – 2) Subtract 2 from 6
8 + 4 12
So, the value of the expression is 12

4. Equations
An equation is a number sentence stating that two amounts are equal. Unlike an expression, an equation does have an equal sign.
17 + c = 52

5. Equations (Example)
Richard had 12 CDs in his collection. He got some more CDs for his birthday. Now he has a total of 15 CDs. What equation can you write to show this?
12 CDs plus some CDs equals 15 CDs
c =
The variable c represents the
number of CDs Richard received
for his birthday.

6. Equations (Example A)
5 tickets plus some extra tickets are 45 tickets. Let t stand for the number of extra tickets.
5 plus the number of extra tickets = 45
t = 45
The equation is 5 + t = 45

7. Equations (Example B)
There are some students. Eight students leave. Now there are 24 students. Let n stand for the beginning number of students.
the beginning number of students – 8 = 24
n – = 24
The equation is n – 8 = 24

8. Solve Equations Mentally
How many CDs did Richard receive for his birthday?
An equation is true if the values on both sides of the equal sign are equal. You solve an equation when you find the value of the variable that makes the equation true. You can solve this equation with mental math.
12 + c = Think: 12 plus what number equals 15?
c = 3
Check: = 15 Replace c with 3
15 = 15  The equation is true.
So, Richard received 3 CDs for his birthday.

9. More Examples 6 + m = 10 Think: 6 plus what number equals 10? m = 4
Check: = 10 Replace m with 4.
10 = 10  The equation checks.
So the value of m is 4.
g – 8 = 3 Think: What number minus 8 equals 3?
g = 11
Check: 11 – 8 = 3 Replace g with 11.
3 = 3  The equation checks
So the value of g is 11.

10. Practice Equations - Write an equation
Practice Equations - Write an equation. Choose a variable for the unknown. Tell what the variable represents.
There are some boxes of cereal on a shelf. Pauline takes 3 boxes. Now there are 12 boxes on the shelf.
There are 15 apples on the table. Five of them are red and the others are green.

11. Practice Equations - Write an equation
Practice Equations - Write an equation. Choose a variable for the unknown. Tell what the variable represents.
There are 25 sodas in a machine. Mr. Lee adds some sodas. Now there are 42 sodas.
Randy has 10 pieces of string. He gave some away. Now he has 3 pieces of string.
There are 25 crayons in a box. Four are red.
The rest are other colors.

12. Practice Equations – Solve the equation by using mental math
Practice Equations – Solve the equation by using mental math. Check your solution.
x + 12 = 17
8 + p = 14
q – 7 = 0
20 – c = 11
3 + k = 3
13 – g = 12
n – 10 = 1
12 + w =14