A cistern has two taps which fill it in 12 minutes and 15minutes respectively. There is also a waste pipe in the cistern. When all the 3 are opened , the empty cistern is full in 20 minutes. How long will the waste pipe take to empty the full cistern?
10 min
20 min
40 min
50 min
Correct Answer :
A. 10 min
Workdone by the waste pipe in 1min
=(1/20)-(1/12)+(1/15) = -1/10 [negative sign means emptying]
therefore the waste pipe will empty the full cistern in 10min