Pipe & Cisterns

Fast trick to solve the pipe and cisterns.

If two pipe 'A' and 'B' operate both at the same time that can fill a tank in 4 hr. A water pipe 'A' fill the tank '4 hr' faster than the pipe 'B' how many hr does it take the pipe 'A' to fill the tank.

STEP 1 :

Let the tank can be filled by pipe 'A' in x hr
than pipe 'B' in 'x + 4 hr'

so in 1hr tank filled by A = 1/x
and in 1hr tank filled by B = 1/(x + 4)


STEP 2 :

Now the equation will be:
1/x + 1/(x + 4) = 1/4

x + 4 + x / x(x + 4) = 1/4

2x + 4 / x2 + 4x= 1/4

8x + 16 = x2 + 4x

so equation will be:

x2-4x-16 = 0


x=6 or x= -2


so the result will be '6 hr' because time must be positive value.


Watch Youtube video of Pipe & Cisterns for better understanding.