The total amount of water in the canteen is 4/4. If you drink after one hour, the remaining amount of water is . If you drink of what was left, you drink *3/4 =3/8. The remaining is 5/8.

First when you drank 1/4 of water, the remaining is 3/4 Then drinking 1/2 of what left -> 1/2 of 3/4 = 3/8 So what will be left after that = 3/4 - 3/8 = 6/8 - 3/8 = 3/8 of water