Question 904053
Form expressions for each rate in LAWNS per MINUTE.  Let x = time for Bobby to mow lawn himself, alone.


RATES
Billy, {{{1/(x-40)}}}
Bobby, {{{1/x}}}
Billy & Bobby Together, {{{1/117}}}


Rate of both together is the sum of their individual rates:
{{{highlight(1/(x-40)+1/x=1/117)}}};
so just solve for x.