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
