Question 554069
Let {{{ x }}} = number of white marbles to be removed
given:
2/3 of the remaining marbles are black
That means 1/3 must be white
{{{ (4 - x) / (8 - x) = 1/3 }}}
{{{ 4 - x = (1/3)*( 8 - x ) }}}
{{{ 3*( 4 - x ) = 8 - x }}}
{{{ 12 - 3x = 8 - x }}}
{{{ -2x = -4 }}}
{{{ x = 2 }}}
If you remove 2 white marbles, then the 
black ones are 4/( 4 + 2 ) = 2/3 of the 
remaining marbles