Question 988944
Let {{{ k }}} = Kenny's age
Let {{{ s }}} = his sister's age
given:
(1) {{{ k = s + 7 }}}
(2) {{{ k + s = 31 }}}
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Substitute (1) into (2)
(2) {{{ s + 7 + s = 31 }}}
(2) {{{ 2s = 24 }}}
(2) {{{ s = 12 }}}
and
(1) {{{ k = s + 7 }}}
(1) {{{ k = 12 + 7 }}}
(1) {{{ k = 19 }}}
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Kenny is 19
His sister is 12 
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check:
(2) {{{ k + s = 31 }}}
(2) {{{ 19 + 12 = 31 }}}
(2) {{{ 31 = 31 }}}
OK