The ratio of the speed of a motor boat in still water to that of the river flow is 5 : 2. If the boat goes along the river flow for 21 hours, then the time (in hours) it will take to come back to its starting point is :

49

The correct answer is Option (3) → 49

Let the speed of the motorboat = $5x$

the speed of the river flow = $2x$

Downstream speed = $5x+2x=7x$

Upstream speed = $5x-2x=37x$

Distance = $7x×21=147x$

∴ Distance upstream = $\frac{147x}{3x}=49$hours