Question

A cistern can be completely drained by a pipe in 6 hours. It can be filled by two pipes in 4 hours and 5 hours, respectively. How many hours will the empty cistern take to fill if all the pipes are running?

If x is the time to fill it, which of the following equations could be used to solve for t?

4x + 5x - 6x = 1
15x + 12x - 10x = 60
4x + 5x = 20

Answer 1

x/4 +x/5 -x/6=1
make the denominator the same: 
15x/60 +12x/60 -10x/60=1
17x=60
x=60/17=3.53 hours

the second choice is the correct answer.