Question 310469
Let filling the cistern be = 1 job.

Both pumps can do the job in 4 hours.
So they can do 1/4 of the job in 1 hour.
.
one pump does the job in 6 hours .
So it does 1/6th. of the job in 1 hour.
.
Job done by both - job done by one of them = job done by the other in 1 hour.
.
1/4 - 1/6 
=3-2 / 12 
=1/12.
The other pump does 1/12th job in 1 hour.
.
So it takes 12 hours to do the complete job of filing the cistern alone.