Question 720033
The rates of any number of workers working together are additive.  Use the idea, rate*time=jobs, the rate being jobs per time.


Will:  1 job per 12 hours, 1/12.
Matthew: r unknown.
Will plus Matthew: 1 job per 7 hours, 1/7.
Find Matthews work rate:  {{{1/7=1/12+r}}}.
{{{r=1/7-1/12}}}
{{{r=(12-7)/84}}}
{{{r=5/84}}}


Using the reciprocal, william would do 5 of these jobs in 84 hours, or the 1 job in {{{0.2*84}}} hours, 16.8 hours. About 16 hours and 45 minutes.