Jane took 15 min to drive her boat upstream to water ski Com

Jane took 15 min to drive her boat upstream to water ski. Coming back later, at the same boat speed, took her 5 min. If the current in that part of the river is 10km/hr, what is her boat speed in still water?

Solution

let the speed of the boat in still water be v km/hr and the one trip distance be d kms
while upstream the net speed will be (v-10)km/hr
time taken to drive upstream = d/(v-10) hrs i.e., = 15 min = 1/4 hrs
and net speed downstream = (v+10)km/hr
time taken to drive downstream = d/(v+10) hrs = 5 min = 1/12 hrs
so, d/(v-10)=1/4 and d/(v+10)=1/12
dividing both the equations
(v+10)/(v-10) = 3/1
so v+10=3v-30
2v=40
thus v=20
boat speed in still water=20km/hr Ans.