10 min

20 min

40 min

50 min

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

7 hrs

7 hrs 30 min

5 hrs

5 hrs 30 min

39 min

40 min

50 min

45min

18 hrs

13 hrs

20 hrs

23 hrs

100 hrs

122 hrs

177 hrs

112 hrs

20 hrs

23 hrs

30 hrs

21 hrs

10 min

20 min

40 min

50 min