9514 1404 393
Answer:
B.
Explanation:
The relation between time, speed, and distance is ...
time = distance/speed
Here, we have a distance of 20 miles. The speed upstream will be 15-c, and the speed downstream will be 15+c. The total time for the two 20-mile trips upstream and back will be ...
20/(15 -c) +20/(15 +c) = 4 . . . . . . matches choice B