Question 990545
Let {{{ a }}} = gallons of gasoline used
Let {{{ b }}} = gallons of substitute used
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I will say that I am filling a 100 gallon
tank with the mixture
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(1) {{{ a + b = 100 }}}
(2) {{{ ( 325a + 90b ) / 100 = 300 }}} ( in cents )
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(2) {{{ 325a + 90b = 30000 }}}
(2) {{{ 65a + 18b = 6000 }}}
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Multiply both sides of (1) by {{{ 18 }}} and
subtract (1) from (2)
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(2) {{{ 65a + 18b = 6000 }}}
(1) {{{ -18a - 18b = -1800 }}}
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{{{ 47a = 4200 }}}
{{{ a = 89.362 }}}
and
(1) {{{ a + b = 100 }}}
(1) {{{ b = 10.638 }}}
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10.638 % of substitute and 89.362 % of gasoline 
must be used
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check answer:
(2) {{{ ( 325a + 90b ) / 100 = 300 }}} 
(2) {{{ ( 325*89.362 + 90*10.638 ) / 100 = 300 }}} 
(2) {{{  29042.65 + 957.42 = 30000 }}}
(2) {{{ 30000.07 = 30000 }}}
error due to rounding off