Question

Two pipes p and q can fill a tank in 3 hrs. and 4 hrs. respectively. if both pipes are opened together, then after what time, should p be closed so that the tank is full in 2 hrs., from the start.

Answer 1

P fills the tank in 3 hrs
P in 1 hr fills : 1/3 part
Q fills the tank in 4 hrs
Q in 1 hr fills : 1/4 part

Pipe 'p' should be closed so that the tank is full in 2 hrs
So pipe Q is opened for 2 hrs.
Pipe Q in 2 hrs fill :2*1/4 = 1/2 part
Remaining = 1 - 1/2 = 1/2 part
P in 1 hr fills== 1/3 part
1 hr == 1/3 part
X===1/2 part
x/3 = 1/2
X = 3/2 = 1.5 hours
So pipe P works for 1.5 hours.
Pipe P has to be closed after 1 hr 30 mins of start or 30 minutes before 2 hours.
Hope it helps. 

Answer 2

Hay!!
Dear user -

We know that in the given question

P fills the tank in=> 3 hrs
P in 1 hr fills => 1/3 part

Q fills the tank in =>4 hrs
Q in 1 hr fills =>1/4 part

Pipe 'p' should be closed
so, that the tank is full in 2 hrs

Then pipe Q is opened for 2 hrs.

Pipe Q in 2 hrs fill 2×1/4
Then pipe Q in fill = 1/2 part
Remaining = 1 - 1/2 = 1/2 part

P in 1 hr fills= 1/3 part
1 hr == 1/3 part
So,
X=>1/2 part

=>x/3 = 1/2

X = 3/2
X= 1.5 hours
So pipe P works for 1.5 hours.

Hence Pipe P has been closed after 1 hr 30 mins of start or 30 minutes before 2 hours.

I Hope it helps.