Question 1005176
Let {{{ a }}} = the 1st person's age
Let {{{ b }}} = the 2nd person's age
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(1) {{{ a = 3b }}}
(2) {{{ a - 15 = 6*( b - 15 ) }}}
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Substitute (1) into (2)
(2) {{{ 3b - 15 = 6*( b - 15 ) }}}
(2) {{{ 3b - 15 = 6b - 90 }}}
(2) {{{ 3b = 75 }}}
(2) {{{ b = 25 }}}
and
(1) {{{ a = 3b }}}
(1) {{{ a = 3*25 }}}
(1) {{{ a = 75 }}}
The 1st person is 75
The 2nd person is 25 
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check:
(2) {{{ a - 15 = 6*( b - 15 ) }}}
(2) {{{ 75 - 15 = 6*( 25 - 15 ) }}}
(2) {{{ 60 = 6*10 }}}
(2) {{{ 60 = 60 }}}
OK