Question 869525
These are each employee's work rates, in JOBS per HOUR:
Employee 1, {{{1/9}}}
Employee 2, {{{1/12}}}
Employee 3, {{{1/13}}}


The combined rate of the three employees working together is the sum of their individual rates.


{{{1/9+1/12+1/13}}}  jobs per hour; but you will want the reciprocal, which will be in hours per job.


The final meaning that you are looking for is {{{1/(1/9+1/12+1/13)}}} HOURS per JOB.