Question

A mechanic can do a job in 2hr. It takes his assistant 3hr to do the same job how long would it take them to complet the job if they worked together?

Answer 1

mechanic' rate --- 1/2
assistant's rate --- 1/3
combined rate = 1/2 + 1/3 = 5/6

time at combined rate = 1/(5/6)
= 6/5 hrs
or 1 hour 20 minutes

Answer 2

oops, 1/5 hr = 12 minutes, not 20 minutes

so 1 hour, 12 minutes