Machine X and Y run at different constant rates. Machine X can complete the job in 9 hours. Machine X works the job for 3 hours and then the two machines work together. They complete the job in 4 more hours. How many hours would it take Y to work the job alone?
X, in 1 hr = 1/9
Y, in 1 hr = 1/Y
x worked for 3 hrs + x,y worked for 4 more = total work done
3*1/9 + 4(1/9+1/Y) = 1
solve for Y