Question 951752
The % concentration is:
( amount of alcohol ) / ( amount of entire solution )
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I can say that there are any amounts I choose
as long as the stated ratio is true, so
I will say:
{{{ 2/3 = 400/600 }}}
Alcohol = {{{ 400 }}} ( say liters for units )
Water = {{{ 600 }}} ( say liters for units )
Alcohol + Water  = {{{ 400 + 600 = 1000 }}} liters
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% concentration of alcohol =
{{{ 400 / 1000 = 40/100 }}}
{{{ 4/100 }}} = 40% alcohol