Question 1009667
Cost of the $18/kg rice: {{{ 18*20 = 360 }}}
Cost of the $16/kg rice: {{{ 16*25 = 400 }}}
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Let {{{ x }}} = the actual value of the mixture 
of the 2 kinds of rice
{{{ ( 360 + 400 ) / ( 20 + 25 ) = x }}}
{{{ x = 760 / 45 }}}
{{{ x = 16.89 }}}
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He asks $19/kg, so he makes {{{ 19 - 16.89 = 2.11 }}}
Since he paid {{{ 18+ 16 = 34 }}} for the rice,
{{{ ( 2.11/34 )*100 = 6.21 }}}
His % gain was 6.21%