2 water pipe inlets pumping water into a tank. Pipe "A" fills up the tank in 30 minutes. Pipe "B" fills up the tank in 90 minutes. Assuming both pipes are on. How long it will take to fill up the tank.
let the capacity of the tank be 1 unit of whatever
rate of 1st pipe = 1/30
rate of 2nd pipe = 1/90
combined rate = 1/30+1/90 = 2/45
time at combined rate = 1/(2/45)
= 45/2
= 22.5 minutes