A boy's speed with the current is 15 km/h and the speed of the current is 2.5 km/h. The boy's speed against the current is :

10 km/h

The correct answer is Option (3) → 10 km/h

Boys speed in water, b

Speed of the current, $c=2.5km/h$

Boys speed with current, $b+c=15km/h$

$⇒b+c=15$

$b=15-2.5=12.5km/h$

∴ Boys speed against current = $b-c$

$=10km/hr$