Question 721109
Look at their rates but in the form of weed-the-garden per minute.


Melinda: 1 job per 40 minutes, or 1/40 jobs per minute.
Melinda plus Bob: 1/15 jobs per minute.


You want the rate for Bob alone?  The rates are additive.  Melinda plus Bob = Melinda+Bob.


{{{highlight(1/40+B=1/15)}}}, where B is the rate at which Bob alone can weed the garden.  This is all, again, jobs per minute.  You are actually looking for the answer to the reciprocal of B.