Question 774951
Let {{{ f }}} = father's age now
Let {{{ s }}} = son's age now
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(1) {{{ f = 3s }}}
(2) {{{ f - 5 = 4*( s - 5 ) }}}
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Substitute (1) into (2)
(2) {{{ 3s - 5 = 4*( s - 5 ) }}}
(2) {{{ 3s - 5 = 4s - 20 }}}
(2) {{{ s = 20 - 5 }}}
(2) {{{ s = 15 }}}
The son is 15
check:
(1) {{{ f = 3*15 }}}
(1) {{{ f = 45 }}}
and
(2) {{{ f - 5 = 4*( s - 5 ) }}}
(2) {{{ 45 - 5 = 4*( 15 - 5 ) }}}
(2) {{{ 40 = 5*10 }}}
(2) {{{ 40 = 40 }}}
OK