Question 136426
Let 1/x= the time of A and B to complete the job. (hours)

rate of A to do the job alone
       A=1/5
rate of B to do the job alone
       B= 1/8

     1/x = 1/5 + 1/8
     
     1/x = 13/40

     13x = 40

       x = 40/13

       x = 3.0769

    therefore A and B can do the job in 3.0769 hours