Question 859663
To quickly get to combined work rate, {{{1/8+1/12=5/24}}}  jobs per hour.


Looking at jobs per dollar,
Fred:  {{{1/(8*10)=1/80}}} jobs per dollar
Sam:  {{{1/(12*7)=1/84}}} jobs per dollar.


BEST COST FOR THE JOB:
To be very simple about finding an answer, not necessarily the perfect best, both of Fred and Sam together  work 24 hours for 5 jobs, so {{{24*10+24*7=608}}} dollars; but that is for FIVE jobs.  For ONE job, {{{608/5=highlight(13.60)}}} dollars.  That is much better than 80 dollars for Fred alone or 84 dollars for Sam alone.