8 girls can do it in 5 days, so 1 girl can do it in 40 days.
So each girl's rate is 1 job per 40 days or  job/day

5 boys can do it in 6 days, so 1 boy can do it in 30 days.
So each boy's rate is 1 job per 30 days, or  job/day.

3 boy's combined rate is  or  jobs/day
6 girl's combined rate is  or  jobs/day.

Let x be the number of days for 3 boys and 6 girls to do the job.

Then in those x days, the boys do x of the job and the girls 
do x of the job.  That must add to 1 job so the equation is

                x + x = 1

Multiply through by 20

                 2x + 3x = 20

                      5x = 20

                       x = 4 days

Edwin