The amount of water in a barrel doubles every minute. Barrel fills up completely in 60 minutes. In how much time will it be half fulled ?

The correct answer is (b).

The amount of water in the barrel doubles every minute. This means that the barrel is half full after 30 minutes.

Option (a) is incorrect because the barrel is not half full after 20 minutes.

Option (c) is incorrect because the barrel is not half full after 40 minutes.

Option (d) is incorrect because the barrel is not half full after 59 minutes.