Question 1081209
Lisa sold b candy bars for d dollars.
You want to find [ dollars ] / [ candy bar ]
which is {{{ d/b }}}, the price of 1 candy bar.
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Jack bought {{{ b }}} candy bars for:
{{{ d/b - .2 }}} dollars each
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In words:
[ Jack's $20 ] / [ Jack's price per candy bar ] = [ number that he can buy ]
{{{ 20 / ( d/b - .2 ) }}}
I can multiply top and bottom by {{{ b }}}
{{{ 20b / ( d - .2b ) }}} = number he can buy
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check answer:
Suppose {{{ b = 4 }}} and {{{ d = 4.8 }}}
{{{ d/b = 1.2 }}}
{{{ d/b - .2 = 1 }}}
Jack's price per candy bar is $1
For $20, he can buy:
{{{ 20b / ( d - .2b ) }}}
{{{ ( 20*4 ) / (  4.8 - .2*4 ) }}}
{{{ 80 / ( 4.8 - .8 ) }}}
{{{ 80 / 4 = 20 }}} bars
OK