Question 832802
This is hard for me to get my head around, also
I picture it a a ballon that looks like it has a 
volume of {{{ 2000 }}} units but it has 1600 units
of a solid material inside it.
{{{ 390 }}} units of the solid material were put inside
without increasing the size of the ballon.
So, there is now {{{ 2000 - 1990 = 10 }}} units of
free space inside
I want to blow up the ballon by {{{ x }}} units so
that ( free space ) / ( total space ) = .2
This also means that ( solid material ) / ( total space ) = .8
I can say ( I think )
{{{ 1990 / ( 1990 + 10 + x ) = .8 }}}
{{{ 1990 / ( 2000 + x ) = .8 }}}
{{{ 1990 = .8*( 2000 + x ) }}}
{{{ 1990 = 1600 + .8x }}}
{{{ .8x = 390 }}}
{{{ x = 487.5 }}}
The container needs to be {{{ 2000 + 487.5  = 2487.5 }}} units
check answer:
( free space ) / ( total space ) = {{{ ( 487.5 + 10 ) / ( 487.5 + 2000 ) }}}
{{{ 497.5 / 2487.5 = .2 }}}
Unless my logic got twisted somehow