Question

34) Two taps A and B can separately fill a cistern in 24 mins and 30 mins respectively. Find
when the pipe B must be turned off, so that the cistern may full in 18 mins?
1) 9 1/2 mins 2) 8 mins
3 7 1/2 mins
4) 6 1/2 mins

Answer 1

A fill the cistern in 24min

B fill the cistern in 30min

Total work =lcm of 24 and 30 =120

Efficiency of A=5

Efficiency of B=4

After B turned off the cistern fill by A in 18 min

So workdone by A is=18 *5=90

Rest work =120-90=30 done by both A and B

Time taken to fill by both A and B=30 /9=3.34min