Question

An inlet pipe can fill a swimming pool in
9 hr, and an outlet pipe can empty the pool in
12 hr. Through an error, both pipes are left
open. How long will it take to fill the pool?

Answer 1

Answer :
       Inlet pipe fills swimming pool in 9 hrs .
     Therefore , part of swimming pool filled by inlet pine in  1 hour = 1/9
  Outlet pine empties swimming pool in 12 hrs .
  therefore , part of swimming pool emptied by outlet pine in 1 hr = 1/12

hence Effective part of swimming pool filled in 1 hr = 1/9 - 1/12 = 12 -9 /9*12
                                                                       =3/108 = 1/36
           therefore 1/36th part of swimming pool is filled in One hour.

Hence it will take 36 hours to fill the swimming pool when both Inlet pipe and outlet pipe are kept Open .

Hope this helps you!!