Question 509695
Let {{{a}}} = Amy's age 
Let {{{t}}} = Tom's age 
Let {{{j}}} = Jill's age 
Let {{{c}}} = Chris's age 
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given:
(1) {{{ a = t + 7 }}}
(2) {{{ t = 2j }}}
(3) {{{ c = 3a }}}
(4) {{{ c + a = 68 }}}
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There are 4 unknowns and 4 equations
so this should be solvable
Substitute (3) into (4)
(4) {{{ c + a = 68 }}}
(4) {{{ 3a + a = 68 }}} 
(4) {{{ 4a = 68 }}}
(4) {{{ a = 17 }}}
Substitute this result into (1)
(1) {{{ a = t + 7 }}}
(1) {{{ 17 = t + 7 }}}
(1) {{{ t = 17 - 7 }}}
(1) {{{ t = 10 }}}
Substitute this result into (2)
(2) {{{ t = 2j }}}
(2) {{{ 10 = 2j }}}
(2) {{{ j = 5 }}}
and, also
(3) {{{ c = 3a }}}
(3) {{{ c = 3*17 }}}
(3) {{{ c = 51 }}}
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Amy's age is 17 
Tom's age  is 10 
 Jill's age is 5 
Chris's age is 51