Question

A motorboat covers the distance between two spots on the river in 8 h and 12 h downstream and upstream respectively. The time required by the boat to cover this distance in still water is

• A. 6.3 h
• B. 9.6 h
• C. 3.2 h
• D. 18.12 h

Answer 1

Answer: (B)

9.6 h

Answer 2

Let u and v be the velocity of boat in still water and velocity of river respectively. If x is the distance between the two spots, then

$u + v = \frac{x}{8}$ (for downstream)

$u - v = \frac{x}{12}$ (for upstream)

On adding

$2u = \frac{20}{96}xoru = \frac{10}{96}x.$

Time required by boat in still water

$= \frac{x}{u} = \frac{x}{10x/96} = 9.6h$