Question 215772
If A can do the job in 2 hours (that's 120 minutes), then A can do {{{1/120}}} of the job in one minute.
If B can do the job in 1 hour(that's 60 minutes) then B can can do {{{1/60}}} of the job in one minute. 
Working together, they can do {{{1/120+1/60 = 1/40}}} of the job in one minute, which means that they can do the entire job in 40 minutes.