Boat travel time calculation

Okay, let's break this down step-by-step:

1. Calculate the downstream speed:

   • When the boat travels downstream (from A to B), its speed is its speed in still water plus the speed of the current.
   • Downstream speed = Boat speed + Water speed
   • Downstream speed = 15 km/h + 3 km/h = 18 km/h

2. Calculate the distance between A and B:

   • We know the downstream speed and the time it took to travel downstream.
   • Distance = Speed × Time
   • Distance (A to B) = 18 km/h × 8 hours = 144 km

3. Calculate the upstream speed:

   • When the boat travels upstream (from B to A), its speed is its speed in still water minus the speed of the current.
   • Upstream speed = Boat speed - Water speed
   • Upstream speed = 15 km/h - 3 km/h = 12 km/h

4. Calculate the time to return from B to A:

   • We know the distance (which is the same as A to B) and the upstream speed.
   • Time = Distance / Speed
   • Time (B to A) = 144 km / 12 km/h = 12 hours

So, it will take the boat 12 hours to return from B to A.