Question 505268
A bath tub can be filled by the hot and cold water pipes in 4 minutes, and when full it can be emptied by the waste pipe in 5 minutes.  If it is half full, and all three pipes are set running, when will the tub be filled?
.
Let t = time (mins) tub is filled
then
t(1/4 - 1/5)= 1/2
multiplying both sides by 20:
t(5 - 4)= 10
t(1)= 10
t = 10 minutes