Question

six taps can fill a cistern in 8 hrs. if 2 taps go out of order find how much more time will be taken?

Answer 1

1 Full Cistern - 6 taps - take - 8 hours

1 Full Cistern - 1 tap - takes -  8 * 6 hours = 48 hours
1 Full cistern - 4 taps - take -  48/4 hours = 12 hours

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another longer method

filling capacity of 6 taps together = 1 Cistern / 8 hours = 1/8 Cistern in 1 hour
filling capacity of 1 tap = 1/8 Cistern / 6  = 1/48 Cistern in one hour

Filling capacity of 4 taps = 1/48 Cistern in 1 hour * 4 = 1/12 Cistern in 1 hour
Time that 4 taps take to fill 1 Cistern = 1 / (1/12) = 12 hours