Question 1031208
What fraction of the fence does Joe get
done in 2 hrs working alone?
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Joe's rate: [ 1 fence built ] / [ 4 hrs ]
{{{ 2*( 1/4 ) = 1/2 }}}
Joe builds 1/2 of the fence in 2 hrs
There is 1/2 of the fence left to build
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Renee joins Joe. They do 1/2 fence 
in {{{ 90 }}} min = {{{ 3/2 }}} hrs
Their rate working together:
[ 1/2 fence ] / [ 3/2 hrs ]
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Let {{{ t }}} = Renee's time working alone 
to finish 1 fence.
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Add their rates working alone to get rate
working together:
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{{{ 1/4 + 1/t = ((1/2)) / ((3/2)) }}}
{{{ 1/4 + 1/t = 1/3 }}}
Multiply both sides by {{{ 12t }}}
{{{ 3t + 12 = 4t }}}
{{{ t = 12 }}}
Working alone, Renee can finish the fence in 12 hrs
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check:
{{{ 1/4 + 1/t = 1/3 }}}
{{{ 1/4 + 1/12 = 1/3 }}} 
{{{ 3/12 + 1/12 = 4/12 }}}
{{{ 4/12 = 4/12 }}}
OK