Question

Two pipes A and B can fill a cistern in 20 minutes and 30 minutes respectively. Both pipes are )
opened, but
pipe B is closed 5 minutes before the cistern is full. How much time is required to fill
the cistem?
(a) 8 minutes
(b) 10 minutes (c) 12 minutes (d) 14 minutes (c) 16 minutes

Answer 1

(A)8 minutes....................................

Answer 2

Answer:

Given that pipe B is closed 5 minutes before the filling of cistern,part of cistern filled by both the pipes in 5 minutes

5×(1/20+1/30)

5×5/60

5×1/12

5/12.

Now part of cistern which remain unfilled

1-5/12

7/12.

The remaining part can be filed by pipe A in 7/12/1/20 minutes

7/12×20

35/3 minutes.

So total time required to fill the cistern

5+35/3

=50/3 which is corresponding to 16 minutes