Question 705286
Let {{{ b }}} = Billy's age
Let {{{ j }}} = Jon's age
Let {{{ d }}} = Dave's age
given:
(1) {{{ d = b + 4 }}}
(2) {{{ b = 3j }}}
(3) {{{ b + j + d = 39 }}}
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(2) {{{ j = b/3 }}}
Substitute (1) and (2) into (3)
(3) {{{ b + j + d = 39 }}}
(3) {{{ b + b/3 + b + 4 = 39 }}}
Now multiply both sides by {{{ 3 }}}
(3) {{{ 3b + b + 3b + 12 = 117 }}}
(3) {{{ 7b = 117 - 12 }}}
(3) {{{ 7b = 105 }}}
(3) {{{ b = 15 }}}
and, since
(1) {{{ d = b + 4 }}}
(1) {{{ d = 15 + 4 }}}
(1) {{{ d = 19 }}}
and
(2) {{{ j = b/3 }}}
(2) {{{ j = 15/3 }}}
(2) {{{ j = 5 }}}
Billy's age is 15
Jon's age is 5
Dave's age is 19
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check answers
(3) {{{ b + j + d = 39 }}}
(3) {{{ 15 + 5 + 19 = 39 }}}
(3) {{{ 39 = 39 }}}
OK