Question

Pump A, working alone, can fill a tank in 3 hours, and pump B can fill the same tank in 2 hrs. If the tank is empty to start and pump A is switched on for one hour, after which pump B is also switched on and the two work together, how many minutes will pump B have been working by the time the pool is filled?

a)48
b)50
c)54
d)60
e)64

Answer 1

pump A, 1/3 tank /hr
pump B, 1/2 tank/hr

in first hour pump A fills 1/3 tank

so together they fill 2/3 tank

x hours (1/3 tank/hr + 1/2 tank/hr) = 2/3 tank

x (5/6) = 2/3 = 4/6

5 x = 4
x = 4/5 hr = (4/5)60 = 48 minutes