Question 938063
The trader bought {{{ 500/x }}} liters of paraffin
He lost {{{ 3 }}} liters, so he now has
{{{ 500/x - 3 }}} liters
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He originally paid {{{ x }}} dollars / liter
He sold for {{{ x+ 1 }}} dollars / liter
He received:
{{{ ( 500/x - 3 )*( x + 1 ) }}}
{{{ 500 - 3x + 500/x - 3 }}}
{{{ 500/x - 3x + 497 }}}
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check: 
suppose {{{ x = 2 }}} dollars
He bought {{{ 500/2 = 250 }}} liters
He lost {{{ 3 }}} liters and has {{{ 247 }}} liters left
He sold for {{{ 3 }}} dollars/liter
{{{ 247*3 = 741 }}} dollars received
and
{{{ 500/x - 3x + 497 }}}
{{{ 500/2 - 3*2 + 497 }}}
{{{ 250 - 6 + 497 = 741 }}}
OK