Question 1063323
Constant Rate of Work Rule  RT=J;
J for amount of jobs.  


Your example uses 1 for how many jobs.


Yolly   {{{(1/9)(jobs/minute)}}}

Marie   {{{(1/60)(jobs/minute)}}}


Marie and Yolly together   {{{(1/9+1/60)(jobs/minute)}}}




Let t be the number of minutes during which both are working together.

{{{highlight((1/9)15+(1/9+1/60)t=1)}}}



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Something seems off in the description.
If Yolly needs 9 minutes to do the whole one job, and if Yolly works alone for FIFTEEN minutes before Marie comes to help, then the amount of this job done BEFORE Marie arrives is   {{{(1/9)*15=(3*5)/(3*3)=5/3>1}}}.
Yolly would have already finished in 9 minutes herself, and would not need to work any more than 9 minutes.