Question

Two points A and B are located 48 km apart on the riverfront. A motorboat must go from A to B and return to A as soon as possible. The river flows at 6 km/h. What must be the least speed of the motorboat in still water for the trip from A to B and back again to be completed in not more than six hours (assume that the motorboat does not stop at B)?

• A. 18 km/h
• B. 16 km/h
• C. 25 km/h
• D. 46 km/h

Answer 1

The correct option is A 18 km/h

Solve through options. At 18 kmph, the motorboat would take exactly 6 hours.
or
Let's take b km/hr as the speed of the motorboat in still water.
Given,
$\frac{48}{(b+6)}+\frac{48}{(b-6)}\le 6$

Solving the above equation, we get the minimum value of b = 18 km/hr.