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Kevin drove from A to B at a constant speed of 60 mph, turned immediately around, and returned at a constant speed of 80 mph. Exactly 4 hours before the end of his trip, he was still approaching B, only 15 miles away from it.
What is the distance between A and B?

A. 275 mi B. 300 mi C. 320 mi D. 350 mi E. 390 mi

Respuesta :

musiclover10045

Let the distance between A and B = d.  

The time from A to B is d/60 and the time from B to A is d/80

The total time for the round-trip would-be: d/60 + d/80 which simplifies to 7d/240.  

Since we are given that exactly 4 hours before the end of his trip, he was still approaching B and was still traveling at 60 mph, only 15 miles away from it, we now have this:

60(7d/240 - 4) = d - 15  

Simplify:

7d/4 - 240 = d - 15  

7d - 960 = 4d - 60  

3d = 900  

Divide both sides by 3:

d = 300  

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