Question 984561
Let {{{ n }}} = the number of boys that 
are needed to join
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Currently, {{{ (1/4)*80 = 20 }}} are boys
{{{ ( 20 + n ) / ( 80 + n ) = 1/3 }}}
{{{ 3*( 20 + n ) = 80 + n }}}
{{{ 60 + 3n = 80 + n }}}
{{{ 2n = 80 - 60 }}}
{{{ 2n = 20 }}}
{{{ n = 10 }}}
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If 10 boys join the club, there will
be 1/3 boys
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check:
{{{ ( 20 + n ) / ( 80 + n ) = 1/3 }}}
{{{ ( 20 + 10 ) / ( 80 + 10 ) = 1/3 }}}
{{{ 30 / 90 = 1/3 }}}
{{{ 1/3 = 1/3 }}}
OK