Ted can mow the lawn in 5 minutes by using his power mower. Galen takes 15 minutes to mow the same lawn using a push-type mower. How many minutes would the job take if the two boys worked together? If t is the time working together, which of the following expressions represents the portion of the job that Ted will complete when the guys work together?

    We need to calculate a TIME.  Here, TIME = job / rate

Here, 

             1 JOB                                      1                 1
------------------------------------- = ------------------ = ---------------
   JOB/5 min + JOB/15 min           1/5 + 1/15     3/15+1/15
           1
= ------------ minutes = 15/4 minutes = 3 3/4 minutes (answer)
          4/15