Question 669866
{{{ a }}} = amount invested @ 8%
{{{ b }}} = amount invested @ 7.2%
given:
(1) {{{ a + b = 24000 }}}
(2) {{{ .08a = (2/3)*.072b }}}
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(1) {{{ b = 24000 - a }}}
Multiply both sides of (2) by {{{ 1000 }}}
(2) {{{ 80a = (2/3)*72b }}}
(2) {{{ 240a = 144b }}}
By substitution:
(2) {{{ 240a = 144*( 24000 - a ) }}}
(2) {{{ 240a = 3456000 - 144a }}}
(2) {{{ 384a = 3456000 }}}
(2) {{{ a = 9000 }}}
and, since 
(1) {{{ b = 24000 - a }}}
(1) {{{ b = 24000 - 9000 }}}
(1) {{{ b = 15000 }}}
$9,000 was invested @ 8%
$15,000 was invested @ 7.2%
check:
(2) {{{ .08a = (2/3)*.072b }}}
(2) {{{ .08*9000 = (2/3)*.072*15000 }}}
(2) {{{ 720 = (2/3)*1080 }}}
(2) {{{ 2160 = 2*1080 }}}
(2) {{{ 2160 = 2160 }}}
OK