Question

A boat covers $6$ km in an hour in still water. It takes thrice as much time in covering the same distance against the current. What is the speed of the current ?

• A. $2$ km/hr
• B. $3$ km/hr
• C. $4$ km/hr
• D. $5$ km/hr

Answer 1

The correct option is C $4$ km/hr

Let the velocity of the boat be $v$ km/hr and velocity of the current be $u$ km/hr

When the boat goes in still water $=v=6$ km/hr

When the boat goes against the current, its velocity is $=v-u=(6-u)$ km/hr

For distance $S$, time taken in still water to cover the distance $=\frac{S}{6}$ hours

For same distance $S$, time taken while going against the current $=\frac{S}{6-u}$ hours

According to given condition,

$\frac{S}{6-u}=3\times \left(\frac{S}{6}\right)$

$\Rightarrow \frac{1}{6-u}=\frac{1}{2}$

$\Rightarrow 6-u=2$

$\Rightarrow u=4$ km/hr