Question 1014044
{{{ x }}} = number of boys
(a)
{{{ 27 - x }}} = number of girls
(b)
{{{ 27 - x = 2x }}}
Add {{{ x }}} to both sides
{{{ 3x = 27 }}}
{{{ x = 9 }}}
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Notice that there must be {{{ 18 }}} girls
and {{{ 9 + 18 = 27 }}}