Bill can hammer 20 nails in 6 minutes. jeff can do the same job in only 5 minutes . how long will it take them to finish if bill hammers the first 5 nails, then jeff hammers for 3 minutes then bill finishes the job?
a. 4.6 minutes
b. 5.0 minutes
c. 5.4 minutes
d. 5.8 minutes
e. 6.0 minutes

Bill hammers 20 nails in 6 minutes.
==> It takes him 6/20 = 0.3 minutes to hammer one nail.
==> If the total job is 20 nails, then he does 1/6 of the job in 1 minute.

Jeff hammers 20 nails in 5 minutes.
==> It takes him 5/20 = 0.25 minutes to hammer one nail.
==> If the total job is 20 nails, then he does 1/5 of the job in 1 minute.

Notice that the question doesn't say what size job they're assigned.
I'll assume it's a 20-nail job.

First step: Bill hammers 5 nails.
At 0.3 minutes per nail, that takes Bill 5 x 0.3 = 1.5 minutes

Second step:  Jeff hammers for 3 minutes.
At 0.25 minute per nail, he adds 3/0.25 = 12 nails to Bill's 5, for a total of 17 so far.

Third step:  Bill finishes the job.
There are (20 - 17) = 3 nails left to go.
At 0.3 minutes per nail, this takes Bill 3 x 0.3 = 0.9 minutes.

First step . . . . .  1.5 minutes
Second step . . . 3    minutes
Third step . . . . . 0.9 minutes
Total . . . . . . . . . 5.4 minutes