A tank can be filled by a tap in 20 min and by another tap in 60 min. Both the taps are kept open for 5 min and then the 1st tap is shut off. After this, how much time the tank will be completely filled?

20 min
30 min
45 min
40 min

Correct Answer: 40 min

Explanation:

Part of the tank filled by both taps in 5 min = 5 x (1/20 + 1/60)
= 5 x (6 +2 )/120 = 8/24 = 1/3
∴ Remaining part = (1 - 1/3) = 2/3
∵ 1/60 Part is now filled in 1 min.
∴ 2/3 Part is now filled in 60 x 2/3 = 40 min.

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