Question 959256
Let {{{ m }}} = Manuel's age now
Let {{{ s }}} = his sister's age now
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given:
(1) {{{ m = s + 8 }}}
(2) {{{ m - 3 = 3*( s - 3 ) }}}
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By substitution:
(2) {{{ s + 8 - 3 = 3*( s - 3 ) }}}
(2) {{{ s + 5 = 3s - 9 }}}
(2) {{{ 2s = 14 }}}
(2) {{{ s = 7 }}}
and
(1) {{{ m = s + 8 }}}
(1) {{{ m = 7 + 8 }}}
(1) {{{ m = 15 }}}
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Manuel is 15
His sister is 7 
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check:
Manuel is 8 years older than his sister
true
3 years ago he was 12
3 years ago, his sister was 4
He was 3 times his sister's age
true