Question 789380
Let {{{ f }}} = father's age now
Let {{{ s }}} = son's age now
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(1) {{{ f = 3s }}}
(2) {{{ f - 12 = 6*( s - 12 ) }}}
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Substitute (1) into (2) 
(2) {{{ 3s - 12 = 6*( s - 12 ) }}}
(2) {{{ 3s - 12 = 6s - 72 }}}
(2) {{{ 3s = 60 }}}
(2) {{{ s = 20 }}}
and, since
(1) {{{ f = 3s }}}
(1) {{{ f = 3*20 }}}
(1) {{{ f = 60 }}}
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The father is 60 and the son is 20
check:
(2) {{{ f - 12 = 6*( s - 12 ) }}}
(2) {{{ 60 - 12 = 6*( 20 - 12 ) }}}
(2) {{{ 48 = 6*8 }}}
(2) {{{ 48 = 48 }}}
OK