Question 935161
Mother rate, 1/90 yards per minute.
Daughter rate, 1/60 yards per minute.
t, time for combined Mother and Daughter raking together, to finish...


{{{(1/90)*15+(1/90+1/60)*t=1}}},  for ONE JOB.


LCD is {{{2*2*3*3*5}}}.


{{{2*15+(2+3)t=180}}}
{{{30+5t=180}}}
{{{5t=150}}}
{{{highlight(t=30)}}}, MINUTES together to finish after Mother works for 15 minutes alone.