Question 884513
Let {{{ w }}} = number of white marbles
Let {{{ r }}} = number of red marbles
Let {{{ b }}} = number of blue marbles
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(1) {{{ w + r + b = 55 }}}
(2) {{{ w = 2r }}}
(3) {{{ b = w + 5 }}}
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(2) {{{ r = w/2 }}}
Substitute (2) and (3) into (1)
(1) {{{ w + w/2 + w + 5 = 55 }}}
(1) {{{ (5/2)*w = 50 }}}
(1) {{{ 5w = 100 }}}
(1) {{{ w = 20 }}}
and
(3) {{{ b = w + 5 }}}
(3) {{{ b = 20 + 5 }}}
(3) {{{ b = 25 }}}
and
(2) {{{ r = w/2 }}}
(2) {{{ r = 20/2 }}}
(2) {{{ r = 10 }}}
Ned has 20 white marbles
check:
(3) {{{ b = w + 5 }}}
(3) {{{ 25 = 20  + 5 }}}
(3) {{{ 25 = 25 }}}
OK