Question 810499
Let {{{ w }}} = ml of water to be added
{{{ 100 / ( 100 + w ) = .2 }}}
{{{ 100 = .2*( 100 + w ) }}}
{{{ 100 = 20 + .2w }}}
{{{ .2w = 80 }}}
{{{ w = 400 }}}
400 ml of water must be added
check:
{{{ 100 / ( 100 + w ) = .2 }}}
{{{ 100 / ( 100 + 400 ) = .2 }}}
{{{ 100/500 = .2 }}}
{{{ 1/5 = .2 }}}
{{{ .2 = .2 }}}
OK