UPSC CAPF AC 2017 Paper

Let, pipe B is closed after x min Then, part filled by pipe A in 1 min =

1

12

and part filled by pipe B in 1 min =

1

16

Now, part filled by pipe (A + B) in x min + part filled by pipe A in (9 - x) min = 1
⇒

x

12

+

x

16

+

9−x

12

=1
⇒

4x+3x+36−4x

48

=1
⇒ 3x+36=48
⇒ 3x=48−36=12
⇒ 3x=12⇒x=4min