Question 627189
Let {{{ a }}} = Jack's age now
Let {{{ b }}} = Jill's age now
given:
(1) {{{ a = b  + 3 }}}
(2) {{{ a + b = 45 }}}
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Substitute (1) into (2)
(2) {{{ b + 3 + b = 45 }}}
(2) {{{ 2b + 3 = 45 }}}
(2) {{{ 2b = 42 }}}
(2) {{{ b = 21 }}}
and, since
(2) {{{ a + b = 45 }}}
(2) {{{ a + 21 = 45 }}}
(2) {{{ a = 24 }}}
Jack is 24 and Jill is 21