Question 1011214
Let {{{ x }}} = the cost of 1 chair in dollars
Let {{{ y }}} = the cost of 1 table in dollars
----------------------------------
given:
(1) {{{ 20x + 2y = 400 }}}
(2) {{{ 8x + 4y = 460 }}}
----------------------
Multiply both sides of (1) by {{{ 2 }}}
and subtract (2) from (1)
(1) {{{ 40x + 4y = 800 }}}
(2) {{{ -8x - 4y = - 460 }}}
----------------------
{{{ 32x = 340 }}}
{{{ x = 10.625 }}}
(1) {{{ 10x + y = 200 }}}
(1) {{{ 10*10.625 + y = 200 }}}
(1) {{{ 106.25 + y =200 }}}
(1) {{{ y = 93.75 }}}
------------------------
check:
(2) {{{ 2x + y = 115 }}}
(2) {{{ 2*10.625 + 93.75 = 115 }}} 
(2) {{{ 21.25 + 93.75 = 115 }}}
and
(1) {{{ 10x + y = 200 }}}
(1) {{{ 10*10.625 + 93.75 = 200 }}}
(1) {{{ 106.25 + 93.75 = 200 }}}
(1) {{{ 200 = 200 }}}
----------------------
{{{ 12x + 3y }}} = cost of 12 chairs and 3 tables
{{{ 12*10.625 + 3*93.75 }}}
{{{ 127.5 + 281.25 = 408.75 }}}
$408.75 is the cost of 12 chairs and 3 tables