Explanation:

P start work first

One hour work=1/8

Q one hour work=1/12

Together one hour work=1/8 +1/12

                     =5/24

4-hour work together=20/24

5 hour work=25/24 it is excess

We take 4-hour work 20/24

Remaining work=4/24

4 hours together means alternative 8 hours

9th hour P turn work is 1/8

Remaining=5/24 -1/8

         =(4-3)/24

         =1/24

10 the turn Q

T(1/12) =1/24

T=1/2 hour

Total time=8+1+1/2

           =9(1/2) hour