A motorboat can travel upsteam on a river at 18km/h and downsteam at 30km/h. How far upstream can the boat travel if it leaves at 8:00am and must return by noon?
if the boat goes upstream for 1km and back, the total time taken is
1/18 + 1/30 = (5+3)/90 = 8/90 hr
since we have 4 hours available, the boat can go
4/(8/90) = 360/8 = 45km up and back.