Bob and ted are in the plumbing business. bob can do the plumbing hook-up for a new house in 4 hours; ted does the same job in 6 hours. if they work together, how long will they take to hook up the plumbing in one house? 2.1 hours 2.4 hours 5 hours

Where 1/4 represents Bob and 1/6 represents Ted...
1/4x + 1/6x = 1
Multiply the two so there is no fraction by the GCF of 12. 
3x + 2x = 12
Add 3x and 2x.
5x = 12
Divide by 5.
x = 12
Plug this value in.
With that, the answer is 2 hours and 24 minutes with x = 12 meaning 144 minutes.