Question 479212
A project can be done by 8 girls in 5 days. 5 boys can do the same project in 6 days. How long will it take for 3 boys and 6 girls to do the job?
<pre>
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 {{{1/40}}} 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 {{{1/30}}} job/day.

3 boy's combined rate is {{{3/30}}} or {{{1/10}}} jobs/day
6 girl's combined rate is {{{6/40}}} or {{{3/20}}} 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 {{{1/10}}}x of the job and the girls 
do {{{3/20}}}x of the job.  That must add to 1 job so the equation is

                {{{1/10}}}x + {{{3/20}}}x = 1

Multiply through by 20

                 2x + 3x = 20

                      5x = 20

                       x = 4 days

Edwin</pre>