Question

Two inlet pipes a and b can fill the tank in 12 minutes and 36 minutes respectively both the pipes are opened together and after sometime pipe a is closed if the tank gets filled in 20 minutes then for how many minutes pipe a was open

Answer 1

for 4min pipe a is open

Answer 2

Given :

Time taken by pipe A = 12 minutes

Time taken by pipe B = 36 minutes

Tank gets filled in 20 minutes and pipe a is closed after some time during the filling.

To find : Pipe A was open for ? minutes.

Solution :

Total number of units =  LCM of 12 and 36 = 36 units

Number of units done by A = $\dfrac{36}{12}=3$

Number of units done by B = $\dfrac{36}{36}=1$

Pipe B was open for 20 minutes .

So, number of units filled by pipe B in 20 minutes = $20\times 1=20$

So, Remaining units would be :

$36-20=16\ units$

So, time for both the pipe to be opened for :

$\dfrac{16}{3+1}=\dfrac{16}{4}=4\ minutes$

Hence, Pipe A was opened for 4 minutes.