If a motorboat moves upstream at the rate of 12 km/hr and downstream at the rate of 18 km/hr, then the speed of the current is:

3 km/hr

The correct answer is Option (4) → 10 km/hr

$\text{Upstream speed} = 12,\quad \text{Downstream speed} = 18$

$\text{Speed of current} = \frac{18 - 12}{2} = 3$

$\text{Speed of current} = 3\ \text{km/hr}$