Question

Isaac drove at an average speed of 49 mph over six hours. During the first three hours he drove at a speed that was 172 mph less than four times the speed he averaged for the last three hours. How fast did he drive during the first three hours?

Answer 1

Issac drove the first three hours at 44 miles per hour.

Step-by-step explanation:

Given,

Average speed of Isaac = 49 mph

Let,

x be the speed during last three hours.

Speed during first three hours = 4x-172

Average = $\frac{Sum\ of\ terms}{No.\ of\ terms}$

$49=\frac{(4x-172)+x}{2}\\\\49=\frac{4x-172+x}{2}\\\\49=\frac{5x-172}{2}$

Multiplying both sides by 2

$49*2=\frac{5x-172}{2}*2\\98=5x-172\\98+172=5x\\270=5x\\5x=270$

Dividing both sides by 5

$\frac{5x}{5}=\frac{270}{5}\\x=54$

Speed during last three hours = x = 54 mph

Speed during first three hours = 4x-172 = 4(54)-172 = 216-172 = 44 mph

Issac drove the first three hours at 44 miles per hour.

Keywords: average, division

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