Upstream speed = ship speed-current speed
Namely:
1/4= ship speed+current speed.
1/6= ship speed-current speed
(Note: "1" in the whole journey)
This is a sum and difference application problem, with a large number = (sum+difference) ÷ 2; Decimal = (sum and difference) ÷2
Solution: water velocity (decimal) = 1/24.
Drift time = 1÷ 1/24=24 (hours)