Three taps A,B and C can fill a cistern in 10, 15 and 20

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Three taps A,B and C can fill a cistern in 10, 15 and 20 minutes respectively. They are all turned on at once, but after 3 minutes C is turned off. How many minutes longer will A and B take to fill the cistern ?

1. 2 min.
2. 2 min. 6 sec.
3. 1 min. 6 sec.
4. 3 min. 8 sec.

Correct Option: B

T = (10 x 15 x 20) / (10 x 15 + 10 x 20 + 15 x 20) = 60/13 minutes
Now, applying the given rule, we have [60/13 x y]/ [y - 60/13 + 3] = 20
or y = 21/10 = 2 min. 6 seconds.