A boat takes 15 hours for travelling D km downstream and (D + 15) km upstream. If the boat takes 13 hours for covering (D – 15) km downstream and (D + 10) km upstream and the speed of stream is 5 km/hr then, find time(in hours) taken by the boat to cover (D + 60) km in downstream?

Correct option is E

Let speed of boat in still water be S kmph
D/(S+5)+(D+15)/(S-5)=15 …………..(1)
(D-15)/(S+5)+(D+10)/(S-5)=13 …………..(2)
From 1 & 2
S=10 & D=45
Required time = (45+60)/(10+5)= 7 hours