The analysis is mainly to understand how to calculate the downstream speed, upstream speed and distance, and the remaining problems will be solved.

Downstream velocity = still water velocity+current velocity

Backward speed = still water speed-water flow speed

Distance = speed × time

Solution: The downstream distance is (a+b)×2.

The distance of counter-current swimming is (a-b)× 1.

So this swimmer swam: (a+b) × 2+(a-b )×1= 3a+b.

If you are satisfied, please adopt it.