A similar question is here:

http://800score.com/forum/viewtopic.php?t=139-----------

In the figure above, car A and car B travel around a circular park with a radius of 20 miles. Both cars leave from the same point, the START location shown in the figure. Car A travels counter-clockwise at 60 miles/hour and car B travels clockwise at 40 miles/hour. Car B leaves 20 minutes after car A. Approximately how many minutes does it take for the cars to meet after car A starts?

A. 63

B. 75

C. 83

D. 126

E. 188

(C) To visualize this question, think about a circle as just a line segment joined at its two ends. Let's first cut the circle at the starting point and make it in to a straight line. Notice that cars A and B are at opposite ends of the line traveling towards each other.

A [ ______________________ ] B

We start by solving for the circumference of the park:

C = 2πr.

Plugging 20 into the equation, we get:

C = 2 × 3.14 × 20 = 125.6 miles.

To calculate how long it takes the cars to meet, use the variable T as the time, in hours, that it takes for the cars to meet after car A starts.

When the cars meet, car A will have traveled 60T miles (distance = rate × time) and car B will have traveled 40 × (T – (1/3)) miles (since it left 20 minutes later, it will have been traveling 1/3 of an hour less). Furthermore, the distance both cars will travel combined is 125.6 miles. So we have the equation:

Total Distance = Distance A travels + Distance B travels

125.6 = 60T + 40(T – (1/3))

125.6 = 100T – (40/3).

Then we have approximately: 139 = 100T. So, T = 1.39 hours.This is equivalent to 1.39 × 60 = 83 minutes.

The correct answer is choice (C).

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**The answer should be 2 × π × 20 = 60 × T + 60 × 1/3 + 40 × T**

T= (2 × π – 1) / 5 Then T should be a little bit bigger than one hour.