9A + 14 (249ml - A) = 13 * 249ml 9A + 3486ml - 14A = 3237ml 5A = 249ml A = 49,8ml <span>B = 249ml - A = 199,2ml </span> Answer: He should use 49,8ml of first brand and 199,2ml of second brand.
Also note: <span>That's an </span>approximation,<span> because </span>the volumes are not strictly additive<span>. For example: mixing 50ml pure ethanol with 50ml water will give you about 95ml of mixture. To get an accurate answer, you'd have to </span>measure<span> the volume of the final mixture and then divide your total alcohol content by </span>that<span>.</span>