Hi Ramya. I'll change the numbers and show you how this kind of thing is solved.

Question: Pipe A takes 15 minutes to fill a cistern and pipe B takes 9 minutes to fill it. How long should you open both pipes before shutting off Pipe A if you want to fill it in exactly 7 minutes?

Solution: Pipe A fills 1/15 of a cistern in a minute and pipe B fills 1/9 of a cistern in a minute. The total time is 7 minutes. All of that time we are running Pipe B. Since rate times time equals volume, Pipe B is contributing:

V = (1/9) x 7 = 7/9 of a cistern.

That means Pipe A must provide the other 2/9 of a cistern. Now we solve for time t:

2/9 = (1/15) t

t = 10/3 = 3 1/3 minutes which is 3 minutes and 20 seconds.

Try it with your numbers.

Cheers,
Stephen La Rocque