1. Write an Equation
COURSE 3 LESSON 7-5
Sound travels at about 1.1  103 ft/s. How far does a sound travel in 11 s?
Sound travels at about 1.1  103 ft/s. Your goal is to find how far a sound will travel in 11 s.
Use the formula d = r • t with the given information.
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2. Write an Equation (continued) Words distance is rate times time
COURSE 3 LESSON 7-5
(continued)
Words distance is rate times time
Let d = the distance the sound travels.
Equation d = 1.1  103 • 11
d = (1.1  103) • 11
= 12.1  103 Multiply 1.1 and 11.
= 1.21  101  103 Write 12.1 in scientific notation.
= 1.21  104 Add the exponents.
The sounds travels 1.21  104 feet.
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3. Write an Equation (continued)
COURSE 3 LESSON 7-5
(continued)
The exponent on 10 for the distance is 4. This is larger than the exponent on 10 for the rate.
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4. Write an Equation
COURSE 3 LESSON 7-5
A certain blood sample contains 3  1012 red blood cells. If there are 5  1010 red blood cells in 1 milliliter of blood, about
how many milliliters of blood are in this sample?
There are 5  1010 red blood cells in 1 milliliter of blood and 3  1012 in the sample. Your goal is to find the number of milliliters of blood in the sample.
You can write an equation to solve this problem.
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5. Write an Equation (continued) Words milliliters of blood
COURSE 3 LESSON 7-5
(continued)
Words
milliliters
of blood
total number of
red blood cells
times is
in a milliliter
Let b = the milliliters of blood.
Equation b • 5  1010 = 3  1012
b (5  1010) = 3  1012
Divide each side by 5.0  1010.
b (5  1010)
5  1010
3  1012 =
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6. Write an Equation  (continued)
COURSE 3 LESSON 7-5
(continued)
Use the Associative Property of Multiplication.
1012
1010 3 5 
b =
Divide. Subtract the exponents.
b = 0.6  102
Write in standard form.
b = 60
There are about 60 milliliters of blood in the sample.
Place the solution in the original equation.
60 (5  1010) = 300  1010 = 3  1012
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7. Write an Equation Write an equation to solve each problem.
COURSE 3 LESSON 7-5
Write an Equation
Write an equation to solve each problem.
1. A space probe travels at 1.5  105 m/h. How far does it travel in 32 days?
2. Light travels at 3  108 m/s. How many seconds does a light ray take to travel 1.8  1011 m?
1.152  108 m
600 s
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