Question

A swimmer can swim $2$ km in $15$ minutes in a lake and in a river he can swim a distance of $4$ km in $20$ minutes along the stream. If a paper boat is put in the river, then the distance covered by it in 2$\frac{1}{2}$2 hours will be

• A. $18$ km
• B. $12$ km
• C. $8$ km
• D. $10$ km

Answer 1

The correct option is D $10$ km

Speed of the man in still water $\frac{2}{15/60}$ = $8$ km/hr
Speed of the man in downstream = $\frac{4}{20/60}$ = $12$ km/hr
Speed of stream = $4$ km/hr
Distance covered by paper boat in $2$ $\frac{1}{2}$ hours = $\frac{5}{2}\times$ $4$ = $10$ km