Find the total time taken by a boat to travel upstream and downstream if the…

Downstream speed equals the boat's still-water speed plus the current speed, and upstream speed equals the boat's still-water speed minus the current speed, because the current adds to the boat's motion in one direction and works against it in the other. For any single leg, time taken equals distance divided by speed, so the total time for a trip that covers the same distance in each direction is the upstream time plus the downstream time, each computed on its own leg's effective speed.

1. Downstream speed = boat speed + current speed = 44 + 28 = 72 km/hr.

2. Upstream speed = boat speed − current speed = 44 − 28 = 16 km/hr.

3. Downstream time = distance ÷ downstream speed = 720 ÷ 72 = 10 hr.

4. Upstream time = distance ÷ upstream speed = 720 ÷ 16 = 45 hr.

5. Total time = downstream time + upstream time = 10 + 45 = 55 hr.

Cross-check with the combined round-trip formula, total time = 2 × D × S ÷ (S2 − C2), where D is the one-way distance, S is the boat's still-water speed, and C is the current speed: 2 × 720 × 44 ÷ (442 − 282) = 63360 ÷ 1152 = 55 hr, which matches the step-by-step result above.

So the total time taken for the round trip is 55 hr.