Two pipes A and B can fill a tank in 36 minutes and 45 minutes respectively. Another pipe C can empty the tank in 30 minutes. First A and B are opened. After 7 minutes, C is also opened. The tank is filled up in

(1) 39 minutes (2) 46 minutes (3) 40 minutes (4) 45 minutes

Solution:

Part of the tank filled by pipes A and B in 1 minute

= 1/36+1/45= 5+4/180

= 9/180 = 1/20

Part of the tank filled by these pipes in 7 minutes

=7/20

Remaining unfilled part

= 1-7/20= 20-7/20= 13/20

When all three pipes are opened .

=1/20-1/30

=3-2/60 = 1/60

:. Time taken in filling 13/20 part

= 13/20*60= 39 minutes

Required time = 39 + 7

= 46minutes

Then Option is (2) 46 Minutes