Question 1091113
Let {{{ a }}} = Stan's age now
Let {{{ b }}} = niece's age now
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(1) {{{ b = (1/3)*a }}}
(2) {{{ b + 12 = (1/2)*( a + 12 ) }}}
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(1) {{{ a = 3b }}}
(2) {{{ 2*( b + 12 ) = a + 12 }}}
(2) {{{ 2b + 24 = a + 12 }}}
(2) {{{ a = 2b + 12 }}}
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Plug (1) into (2)
(2) {{{ 3b = 2b + 12 }}}
(2) {{{ b = 12 }}}
and
(1) {{{ a = 3*12 }}}
(1) {{{ a = 36 }}}
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Stan is 36 now
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check:
(2) {{{ b + 12 = (1/2)*( a + 12 ) }}}
(2) {{{ 12 + 12 = (1/2)*( 36 + 12 ) }}}
(2) {{{ 24 = (1/2)*48 }}}
(2) {{{ 24 = 24 }}}
OK