posted by Sonia
if you do a job in 4 hours, and your friend can do the same job but in 6 hours, and if you both work together how long will it take to complete the job?
your rate = job/4
friend's rate = job/6
combined rate = job/4 + job/6 = 5job/6
Time = job/rate = job/(5job/12))
= 12/5 hours or 2 hours and 24 minutes.