Question 1032377
<pre>
Painter        Rate (house per hours)   Alternative variable rate

Abe           {{{1/a}}}                      A

Bart          {{{1/b}}}                      B

Cain          {{{1/c}}}                      C

Abe&Bart     {{{1/15}}}

Bart&Cain    {{{1/12}}}

Abe&Cain     {{{1/20}}}
</pre>

You might choose to form a linear system in three variables using the alternative named variables.
{{{system(A+B=1/15,B+C=1/12,A+C=1/20)}}}


What what you know, maybe substitution being the easier method.