Question 252103
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.