[不恥下問系列]又有GMAT數學題不懂之17

Assume :
the volume of the pool is V m^3
the constant rate of pump A be x m^3/h
then the constant rate of pump B be 1.5x m^3/h
Since :
Two water pumps, working simultaneously at their respective constant rates, took exactlyu 4 hours to fill a certain swimming pool
By time=volume/rate
4=V/(1.5x+x)
V/x=10
The time reqired for the faster pump to fill the pool if it had worked alone at its constant rate
=V/(1.5x)
=(1/1.5)(V/x)
=(10/1.5)
=20/3 hour
Ans :E

let x be the rate of the slower pump

rate of faster pump = 1.5x

volume of the pool = (1.5x + x) * 4 hrs = 10x

time required to fill up the pool with faster pump

= 10x / 1.5x = 20 / 3

answer = E

1/[(1/x+1/(1.5x)]=4
3x/(3+2)=4
3x=20
x=20/3
(c)