Question 831097
Let {{{ a }}} = the price he originally paid for the 
chair that he made a 20% profit on
let {{{ b }}} = the price he originally paid for the 
chair that he made a 12% profit on
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Equation for the 1st chair:
{{{ a + .2a = 500 }}}
(1) {{{ 1.2a = 500 }}}
Equation for the 2nd chair:
{{{ b - .12b = 500 }}}
(2) {{{ .88b = 500 }}}
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(1) {{{ a = 500/1.2 }}}
(1) {{{ a = 416.67 }}}
and
(2) {{{ b = 500/.88 }}}
(2) {{{ b = 568.18 }}}
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he paid a total of rs {{{ 416.67 + 568.18 = 984.85 }}}
He sold them for a total of rs {{{ 500 + 500 = 1000 }}}
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His profit was rs {{{ 1000 - 984.85 = 15.15 }}}
His profit  percent was {{{ ( 15.15/984.85 )*100 = 1.54 }}}
1.54%
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You can check my math. I believe my method is OK