Question 38574
We use the work problem formula
t/A + t/B = 1
t = time worked together
A = first guy's time alone
B = second guy's time alone
1 = one job or task
so plug in and get
3/4 + 3/B = 1
multiply all by 4B and get
3B + 12 = 4B
and 
B = 12 hours