Question 723565
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Here it is broken down so you can understand where everything comes from.
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It takes Jeff 90 mins. to mow the yard. 
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Therefore Jeff's mowing rate is 1 lawn per 90 minutes or {{{1_lawn/90_min}}} or {{{1/90}}}{{{lawn/min}}}
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It takes Joe 105 mins to mow the yard.
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Therefore Joe's mowing rate is 1 lawn per 105 minutes or {{{1_lawn/105_min}}} or {{{1/105}}}{{{lawn/min}}}
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What is the approximate time it will take if they work together to mow the yard?
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Let x be the answer, Then it takes them x mins to mow the yard working
together.
Therefore their combined mowing rate is 1 lawn per x minutes or {{{1_lawn/x_min}}} or {{{1/x}}}{{{lawn/min}}} 

The equation comes from:

                  {{{(matrix(7,1,

"JEFF'S",MOWING,RATE,IN,LAWNS,PER,MINUTE))}}}{{{""+""}}}{{{(matrix(7,1,

"JOE'S",MOWING,RATE,IN,LAWNS,PER,MINUTE))}}}{{{""=""}}}{{{(matrix(8,1,

THEIR,COMBINED,MOWING,RATE,IN,LAWNS,PER,MINUTE))}}}

                      {{{1/90}}}{{{lawn/min}}}{{{""+""}}}{{{1/105}}}{{{lawn/min}}}{{{""=""}}}{{{1/x}}}{{{lawn/min}}}

                             {{{1/90}}}{{{""+""}}}{{{1/105}}}{{{""=""}}}{{{1/x}}}

Solve that by multiplying through by 630x

Answer {{{48&6/13}}} minutes.


Edwin</pre>