Question 1041930
<pre><b>
Their rates are {{{matrix(1,2,1,attic)/matrix(1,2,15,hours)}}} and {{{matrix(1,2,1,attic)/matrix(1,2,13,hours)}}}

Their combined rate is the sum of their rates {{{1/15+1/13}}}. 

Since {{{TIME=matrix(1,3,NUMBER,OF,JOBS)/matrix(1,3,THEIR,COMBINED,RATE)}}}

Then since they are to do 1 job, the number of jobs is 1, and

{{{1/(1/15+1/13)}}}

Multiply top and bottom by LCD = 15&#8729;13=195

{{{1*195/((1/15+1/13)*195)}}}

{{{195/(195/15+195/13)}}}

{{{195/(13+15)}}}

{{{195/28}}}

{{{matrix(1,2,6&27/28,hours)}}}

Edwin</pre></b>