Question 562871
Let {{{ s }}} = Sue's age
Let {{{ e }}} = Emma's age
Let {{{ a }}} = Abby's age
given:
(1) {{{ s = 2e }}}
(2) {{{ s = a + 7 }}}
(3) {{{ s + e + a = 53 }}}
This is 3 equations and 3 unknowns, 
so it's solvable
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(2) {{{ s = a + 7 }}}
(2) {{{ a = s - 7 }}}
and
(1) {{{ e = s/2 }}}
Substitute (1) and (2) into (3)
(3) {{{ s + s/2 + s - 7 = 53 }}}
(3) {{{ (5/2)*s - 7 = 53 }}}
(3) {{{ (5/2)*s = 60 }}}
(3) {{{ 5s = 120 }}}
(3) {{{ s = 24 }}}
and
(2) {{{ a = s - 7 }}}
(2) {{{ a = 24 - 7 }}}
(2) {{{ a = 17 }}}
and
(1) {{{ e = s/2 }}}
(1) {{{ e = 24/2 }}}
(1) {{{ e = 12 }}}
Sue's age is 24
Emma's age is 12
Abby's age is 17
check:
(3) {{{ 24 + 12 + 17 = 53 }}}
(3) {{{ 53 = 53 }}}
OK