Question 911303
One key thing to keep in mind is
that the difference in their ages never changes
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Let {{{ s }}} = son's age now
Let {{{ f }}} = father's age now
{{{ s + 20 }}} = son's age in 20 years
{{{ f + 20 }}} = father's age in 20 years
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(1) {{{ f = 10s }}}
(2) {{{ f + 20 = 2*( s + 20 ) + 4 }}}
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Substitute (1) into (2)
(2) {{{ 10s + 20 = 2*( s + 20 ) + 4 }}}
(2) {{{ 10s + 20 = 2s + 40 + 4 }}}
(2) {{{ 8s = 24 }}}
(2) {{{ s = 3 }}}
and
(1) {{{ f = 10s }}}
(1) {{{ f = 10*3 }}}
(1) {{{ f = 30 }}}
The difference in their ages is 
{{{ 30 - 3 = 27 }}} , so
the father was 27 when his son was born
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check:
(2) {{{ 10s + 20 = 2*( s + 20 ) + 4 }}}
(2) {{{ 10*3 + 20 = 2*( 3 + 20 ) + 4 }}}
(2) {{{ 30 + 20 = 2*23 + 4 }}}
(2) {{{ 50 = 46 + 4 }}}
(2) {{{ 50 = 50 }}}
OK