Question 863598
Roofer rate, 1/12 jobs per hour
Assistant's rate, 1/u jobs per hour


Equation for the description:
Using R*T=J  rate time job,
{{{(1/12+1/u)*3+(1/u)*12=1}}}
That is the sum of the two work situations.  One expression for both working together three hours, and the other for just the assistant working alone for 12 hours.


Solve for u.  
{{{3/12+3/u+12/u=1}}}
{{{1/4+15/u=1}}}
.
{{{u+60=4u}}}
{{{60=3u}}}
{{{highlight(u=20)}}} hours for the assistant if doing 1 job alone.