Question 282711
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For a formal algebraic solution using the standard method for solving 2-part mixture problems like this, see the response from tutor @ikleyn.<br>
If formal algebra is not required, the answer can be obtained quickly with some mental arithmetic, using the fact that the ratio in which the two ingredients need to be mixed is exactly determined by where the target percentage lies between the percentages of the two ingredients.<br>
Having used a lot of words to explain how this method works, here is the short work needed to get the answer.<br>
(1) use some mental arithmetic to find that 45% is "3 times as close" to 30% as it is to 90%;
(2) that means the mixture needs to contain 3 times as much of the 30% acid as it does the 90% acid<br>
Since the total amount is 4 gallons, you need 3 gallons of the 30% acid and 1 gallon of the 90% acid<br>
ANSWER: 3 gallons of the 30% acid, 1 gallon of the 90% acid<br>