Question 867384
Let {{{ b }}} = the number of boys at first
Let {{{ g }}} = the number of girls at first
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(1) {{{ b = 4g }}}
(2) {{{ g = 2*( b - 105 ) }}}
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Substitute (1) into (2)
(2) {{{ g = 2*( 4g - 105 ) }}}
(2) {{{ g = 8g - 210 }}}
(2) {{{ 7g = 210 }}}
(2) {{{ g = 30 }}}
and, since
(1) {{{ b = 4g }}}
(1) {{{ b = 4*30 }}}
(1) {{{ b = 120 }}}
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The number of children 
at first = {{{ b + g }}}
{{{ b + g = 120 + 30 }}}
{{{ b + g = 150 }}}
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check answer:
(2) {{{ g = 2*( b - 105 ) }}}
(2) {{{ 30 = 2*( 120 - 105 ) }}}
(2) {{{ 30 = 2*15 }}}
(2) {{{ 30 = 30 }}}
OK