Question 903343
Let {{{ w }}} = ml of water to be added
{{{ .8*400 = 320 }}} ml of alcohol in 80% solution
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{{{ 320 / ( 400 + w ) = .5 }}}
{{{ 320 = .5*( 400 + w ) }}}
{{{ 320 = 200 + .5w }}}
{{{ .5w = 120 }}}
{{{ w = 240 }}}
240 ml of water should be added
check:
{{{ 320 / ( 400 + 240 ) = .5 }}}
{{{ 320 / 640 = .5 }}}
{{{ .5 = .5 }}}
OK