Question 841120
Let {{{ g }}} = the number of girls
{{{ y - g }}} = the original number of boys
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given:
{{{ ( y - g + 20 ) / g = 5/7 }}}
Multiply both sides by {{{ 7g }}}
{{{ 7*( y - g + 20 ) = 5g }}}
{{{ 7y - 7g + 140 = 5g }}}
{{{ 7y = 12g - 140 }}}
{{{ 12g = 7y + 140 }}}
{{{ g = (7/12)*y + 35/3 }}}