A man’s speed with the current is 15 km/h and the current’s speed is 2.5 km/h. What is his speed against the current?

Concept / Approach

• With current: v_b + v_s; Against current: v_b − v_s.

Step-by-step calculationv_b + v_s = 15, with v_s = 2.5Boat speed in still water v_b = 15 − 2.5 = 12.5Against current = 12.5 − 2.5 = 10 km/h

Final Answer10 km/h