three taps a, b and c can fill a tank in 12, 15 and 20 hours respectively. if a is open all the time and b and c are open for one hour each alternately, the tank will be full in---------?

7 hours

6 2/3 hours

6 hours

7 1/2 hours

three taps a, b and c can fill a tank in 12, 15 and 20 hours respectively. if a is open all the time and b and c are open for one hour each alternately, the tank will be full in---------?

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