Question 1039218
Let {{{ x }}} = grams of nuts @ P 1.00 / gram 
he must sell to average 0.85/gram
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{{{ .85 = ( .75*15 + 1*x ) / ( 15 + x ) }}}
{{{ .85*( 15 + x ) = 11.25 + x }}}
{{{ 12.75 + .85x = 11.25 + x }}}
{{{ .15x = 1.5 }}}
{{{ x = 10 }}}
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He must sell 10 grams of nuts @ 1.00 / gram
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check:
{{{ .85 = ( .75*15 + 1*x ) / ( 15 + x ) }}}
{{{ .85 = ( .75*15 + 1*10 ) / ( 15 + 10 ) }}}
{{{ .85 = ( 11.25 + 10 ) / 25 }}}
{{{ 21.25 = 21.25 }}}
OK