Question 896878
Try assigning a variable to one of the times for one of the workers to do 1 lawn-mow job.  Mindy seems to be best for this time variable assignment.


Alfred, {{{1/(2x)}}}
Mindy,  {{{1/x}}}
Casey, {{{1/(x+10)}}}
The time unit for x is MINUTES.


The three working all at once do the job in 60 minutes, is the sum of their individual rates.
{{{highlight_green(1/(2x)+1/x+1/(x+10)=1/60)}}}.
Just a bit of algebra to solve for x.


Note very carefully, the simplest common denominator is {{{60*x*(x+10)}}}.
Be sure you know why, and how.