Question 1019883
DAVID CAN WASH A SINK FULL OF DISHES IN 45 MINUTES, 
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David's dishwashing rate is 1 sinkful per 45 minutes or

{{{matrix(1,2,1,SINKFUL)/matrix(1,2,45,MINUTES)}}} or {{{matrix(1,2,1/45,SINKFUL/MINUTE)}}} 
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WHILE TODD CAN WASH THE SAME AMOUNT IN 30 MINUTES.
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Todd's dishwashing rate is 1 sinkful per 30 minutes or

{{{matrix(1,2,1,SINKFUL)/matrix(1,2,30,MINUTES)}}} or {{{matrix(1,2,1/30,SINKFUL/MINUTE)}}}
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HOW LONG WILL IT TAKE THEM TO WASH THE DISHES IF THEY DO IT TOGETHER ?
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Suppose it takes them x minutes working together.

Then their combined dishwashing rate is 1 sinkful per x minutes or

{{{matrix(1,2,1,SINKFUL)/matrix(1,2,x,MINUTES)}}} or {{{matrix(1,2,1/x,SINKFUL/MINUTE)}}}

The equation comes from:

{{{matrix(3,1,"DAVID'S",DISHWASING,RATE)}}}{{{""+""}}}{{{matrix(3,1,"TODD'S",DISHWASING,RATE)}}}{{{""=""}}}{{{matrix(4,1,THEIR,COMBINED,DISHWASING,RATE)}}}

{{{1/45+1/30=1/x}}}

Solve that for x.

Edwin</pre>