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)