Question 317606
1/12+1/x=1/(x-1)
1/12=1/(x-1)-1/x
1/12=(x-(x-1)/x(x-1)
1/12=-1/(x^2-x)
x^2-x=-12
x^2-x+12=0
(x-4)x+3)=0
x-4=0 
x=4 hrs. for the second pipe to fill the tank working alone.
Proof:
1/12+1/4=1/(4-1)
1/12+1/4=1/3
1/12+3/12=1/3
4/12=1/3
1/3=1/3