A boat takes 8 hrs to travel certain distance in a river in down stream and it takes 10 hrs to travel the same distance in upstream. Then the time taken by the boat to travel the same distance in still water is

8=dVb+Vw;10=dVb−Vw             And t=dVb             ∴ Vb+Vw=d8             Vb−Vw=d10             ⇒2Vb=d[18+110]             ⇒2Vb=d×1880             ⇒dVb=16018             ⇒t=8.8hr