Question 1058227
Let the sister's age be
{{{ a }}}, {{{ b }}}, and {{{ c }}}
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(1) {{{ a + b + c = 78 }}}
(2) {{{ b = 2a - 18 }}}
(3) {{{ c = a + 8 }}}
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Substitute (2) and (3) into (1)
(1) {{{ a + b + c = 78 }}}
(1) {{{ a + 2a - 18 + a + 8 = 78 }}}
(1) {{{ 4a - 10 = 78 }}}
(1) {{{ 4a = 88 }}}
(1) {{{ a = 22 }}}
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(2) {{{ b = 2a - 18 }}}
(2) {{{ b = 2*22 - 18 }}}
(2) {{{ b = 44 - 18 }}}
(2) {{{ b = 26 }}}
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(3) {{{ c = a + 8 }}}
(3) {{{ c = 22 + 8 }}}
(3) {{{ c = 30 }}}
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The sisters are 22, 26, and 30 
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check:
(1) {{{ a + b + c = 78 }}}
(1) {{{ 22 + 26 + 30 = 78 }}}
(1) {{{ 78 = 78 }}}
OK