John can finish a certain job in three hours and chris can finish it in two hours. how many hours will it take them to finish the job if they work together ?

Here is a chart you can make
Rate= (you can figure this out because in 1 hr he does 1/3 of the work)
                Work       Time        Rate
John            1               3         1/3
Chris            1              2          1/2
Together     1            6/5            5/6
combine the rates because they both will do 5/6 of the work in 1 hour. then find the reciprocal so it equals one work

The answer is 1 and 1/5hr or 12 min.