A motor boat can row at the speed of 12 km/h in still water. If the river is flowing at 4 km/h and it takes 12 hours for a round trip, then the distance the two places is :

64 km

The correct answer is Option (3) → 64 km

Total time for the round trip, $T_{total}$

$T_{total}$ = Time for downstream + Time for upstream

$12=\frac{d}{16}+\frac{d}{8}$

$⇒3d=192$

$⇒d=64km$