A cistern can be filled by taps a,b in 12 hrs and 6 hrs respectively.The full cistern can be emptied by a third tap c in 8 hrs.If all the taps be turned on at the same time, in how much time will the empty cistern be filled completly?
(a)48 hrs
(b)40 hrs
(c)36 hrs
(d)24 hrs

Lets us choose to work with a 24 L tank, as the size does not matter.
I choose 24 as 24 = LCF(6,8,12)

tap A adds at 2L per hour
tap B adds at 4L per hour
tap C empties at 3 L per hour.

net effect is 2 + 4 - 3 = add 3 L per hour

hence it will take 24 / 3 = 8 hours

cheers