Question 919307
Three brothers,Michael,Matthew and Mark are tasked to mow a 2000 square meter garden... 
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In 8 hours, Michael...can mow 1200 square meters. 
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So Michael's mowing rate is 1200 square meters per 8 hours or 

{{{matrix(1,3,1200,square,meters)/matrix(1,2,8,hours) }}}{{{""=""}}}{{{matrix(1,2,1200/8,matrix(1,2,square,meters)/hour)}}}{{{""=""}}}{{{matrix(1,2,150,matrix(1,2,square,meters)/hour)}}}
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In 8 hours...Matthew can mow 600 square meters
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So Matthew's mowing rate is 600 square meters per 8 hours or 

{{{matrix(1,3,600,square,meters)/matrix(1,2,8,hours) }}}{{{""=""}}}{{{matrix(1,2,600/8,matrix(1,2,square,meters)/hour)}}}{{{""=""}}}{{{matrix(1,2,75,matrix(1,2,square,meters)/hour)}}}
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In 8 hours...Mark...can mow 200 square meters.
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So Mark's mowing rate is 200 square meters per 8 hours or 

{{{matrix(1,3,200,square,meters)/matrix(1,2,8,hours) }}}{{{""=""}}}{{{matrix(1,2,200/8,matrix(1,2,square,meters)/hour)}}}{{{""=""}}}{{{matrix(1,2,25,matrix(1,2,square,meters)/hour)}}}
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If they work together for 5 hours,
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Their combined mowing rate is {{{150+75+25}}} or {{{matrix(1,2,250,matrix(1,2,square,meters)/hour)}}} 

Therefore in 5 hours they will have mowed {{{250*5}}} or {{{matrix(1,3,1250,square,meters)}}}
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what part of the garden is not mowed?
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...a 2000 square meter garden.
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So 2000-1250 = 750 square meters are no mowed.

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Follow up question: with reference to the question above, suppose the three brothers worked together for 4 hours...
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Their combined mowing rate is  {{{matrix(1,2,250,matrix(1,2,square,meters)/hour)}}} 

Therefore in 4 hours they will have mowed {{{250*4}}} or {{{matrix(1,3,1000,square,meters)}}}
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but Mark got tired and quit. Matthew stayed for another half-hour and quit.
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Michael's and Matthew's combined mowing rate is {{{150+75}}} or {{{matrix(1,2,225,matrix(1,2,square,meters)/hour)}}}.

Therefore in that half hour Michael and Matthew will have mowed {{{225*(1/2)}}} or {{{matrix(1,3,112.5,square,meters)}}}

So when Matthew quits, the number of square meters mowed is 1000+225+112.5 or
1337.5 square meters.

So there are 2000-1337.5 or 662.5 square meters unmowed. 
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How long will it take to Michael finish the job?
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Since Michael's rate is {{{matrix(1,2,150,matrix(1,2,square,meters)/hour)}}},
and {{{TIME}}}{{{""=""}}}{{{PRODUCTION/RATE}}}, it will take him {{{662.5/150=6625/1500=53/12=4&5/12}}} or 4 hours and 25 minutes.

Edwin</pre>