The road connecting two mountain villages goes only uphill or downhill. A bus always travels 15 mph uphill and 30 mph downhill. Find the distance between the villages if it takes exactly four hours for the bus to complete a round trip.

Answer 1

time(up) = distance/rate = d/15
time(down) = d/r = d/30
Add time up to time down to get
(d/15)+(d/30) = total time = 4.
Solve for d.
[Remember: time(up) + time(down) = 4.0 hr]