Question 855484
Let {{{ a }}} = father's age now
Let {{{ b }}} = Son's age now
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(1) {{{ a + b = 36 }}}
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Subtract {{{ 2b }}} from both sides
(1) {{{ a - b = 36 - 2b }}}
and
(1) {{{ a = 36 - b }}}
When the Son's age is {{{ 36 - b }}} ( father's age now )
the difference in their ages will still be the same
( it never changes ), so
{{{ 36 - b + ( 36 - b + ( 36 - 2b ) )  = 84 }}}
{{{ 36 - b + 36 - b + 36 - 2b = 84 }}}
{{{ 3*36 - 4b = 84 }}}
{{{ 108 - 4b = 84 }}}
{{{ 4b = 108 - 84 }}}
{{{ 4b = 24 }}}
{{{ b = 6 }}}
and
(1) {{{ a = 36 - b }}}
(1) {{{ a = 30 }}}
The father is 30
The son is 6
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check:
In 24 years, the son will be 30 
In 24 years, the father will be 54
{{{ 30 + 54 = 84 }}}
OK