Question 1024681
Let {{{ s }}} = son's age now
Let {{{ f }}} = father's age now
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(1) {{{ s + f = 50 }}}
(2) {{{ f + 5 = 2*( s + 5 ) }}}
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It's possible to solve this using just one
variable such as {{{ x }}}, but I think it's 
more clear to do it this way. 
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(1) {{{ f = 50 - s }}}
Substitute (1) into (2)
(2) {{{ 50 - s + 5 = 2*( s + 5 ) }}}
(2) {{{ 55 - s = 2s + 10 }}}
(2) {{{ 3s = 45 }}}
(2) {{{ s = 15 }}}
and
(1) {{{ f = 50 - s }}}
(1) {{{ f = 50 -15 }}}
(1) {{{ f = 35 }}}
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The father is 35 and
the son is 15
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check:
(2) {{{ f + 5 = 2*( s + 5 ) }}}
(2) {{{35 + 5 = 2*( 15 + 5 ) }}}
(2) {{{ 40 = 2*20 }}}
(2) {{{ 40 = 40 }}}
OK