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Paul drives from his apartment to his parents' house and

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Paul drives from his apartment to his parents' house and

by swerve » Mon Jun 17, 2019 1:32 pm

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Paul drives from his apartment to his parents' house and back. On the trip to his parents' house, he travels at an average speed of 60 miles per hour. On the return trip, Paul drives at an average speed of 80 miles per hour. Which of the following is the closest approximation of Paul's average speed, in miles per hour, for the round trip?

A. 60
B. 68.6
C. 70.0
D. 71.4
E. 80.0

The OA is B

Source: Princeton Review
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by Scott@TargetTestPrep » Wed Jun 19, 2019 5:33 pm
swerve wrote:Paul drives from his apartment to his parents' house and back. On the trip to his parents' house, he travels at an average speed of 60 miles per hour. On the return trip, Paul drives at an average speed of 80 miles per hour. Which of the following is the closest approximation of Paul's average speed, in miles per hour, for the round trip?

A. 60
B. 68.6
C. 70.0
D. 71.4
E. 80.0

The OA is B

Source: Princeton Review
We can let the one-way distance be 240 miles. Therefore, the average speed of the round trip is:

(240 + 240)/(240/60 + 240/80)

480/(4 + 3)

480/7 ≈ 68.6 mph

Answer: B

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by deloitte247 » Sun Jun 23, 2019 4:22 am
$$Average\ speed\ for\ whole\ trip=\frac{\left(Total\ di\tan ce\right)}{Total\ time}$$
Since distance is equal;
$$=\frac{D}{\frac{D}{60}}+\frac{D}{\frac{D}{80}}$$
$$=\frac{2D}{\frac{4D+3D}{240}}\ \ \ \ =\frac{2D}{\left(\frac{7D}{240}\right)}$$
$$=\frac{\left(2D\cdot240\right)}{7D}$$
$$=\frac{480}{7}\ \ \ \ \ =68.57\ mph$$

The answer to this question is B.
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