Word Problem with Fractions

Date: 2/15/96 at 20:41:43
From: sigit
Subject: finishing the job

Jack can finish a job in 5 days, and Richard can finish the same 
job in 7 days.  A job is to be done by Jack and Richard together.  
How long will it take them to finish this job ?

Date: 2/20/96 at 23:29:53
From: Doctor Byron
Subject: Re: finishing the job

We can reword this to say that
Jack does 1/5 of a job in a day, and
Richard does 1/7 of a job in a day.

Together they complete 1/5 + 1/7 = 12/35 of a job in a day.

The time it will take them to complete the job is simply the 
reciprocal of this value.  (Just as we took the reciprocal to find
the values above.)

Therefore the job will take them 35/12 = 2 11/12 days.

I hope this has helped.  Good luck.

-Doctor Byron,  The Math Forum