Question 624555
Let {{{ a}}} = Steven's age
Let {{{ b }}} = Sarah's age
given:
(1) {{{ a = b + 13 }}}
(2) {{{ a + b = 28 }}}
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Substitute (1) into (2)
(2) {{{ b + 13 + b  = 28 }}}
(2) {{{ 2b + 13 = 28 }}}
Subtract {{{ 13 }}} from both sides
(2) {{{ 2b = 28 - 13 }}}
(2) {{{ 2b = 15 }}}
(2) {{{ b =7.5 }}}
and, since
(1) {{{ a = b + 13 }}}
(1) {{{ a = 7.5 + 13 }}}
(1) {{{ a = 20.5 }}}
Steven is 20 yrs 6 mos old
Sarah is 7 yrs 6 mos old
check answer:
Do ages add up to {{{ 28 }}} ?
Is the difference {{{ 13 }}}  ?
Yes