Question 892275
RT=J for rate time job.
Uniform rates for job or task completion.
Rate is in units of jobs per days.


Let d = time for Rob to do 1 job;
Rob, 1/d
George, 1/(3d)
Rob&George, 1/12
Sarah, 1/(6d)


Think very carefully about those rate expressions and how they conform to the description before continuing.


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The combined work rate for Rob and George is the sum of their rates.
{{{highlight_green(1/d+1/(3d)=1/12)}}}
{{{3/(3d)+1/(3d)=1/12}}}
{{{4/(3d)=1/12}}}
{{{3d/4=12}}}
{{{3d=36}}}
{{{highlight(d=12)}}}


With d now known, the rates for Rob and George can be evaluated.  The time for Sarah to do 1 job is {{{highlight(6d=highlight(72))}}} days.