Question 1198982
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If a formal algebraic solution is not required, here is a quick and easy way to solve any 2-part mixture problem like this.<br>
Look at the three percentages 5, 10, and 25 on a number line and observe/calculate that 10% is 5/20 = 1/4 of the way from 5% to 25%.  That means 1/4 of the mixture has to be the 25% solution.<br>
1/4 of 500 ml is 125 ml, so the mixture needs to be 125 ml of the 25% solution and 375 ml of the 5% solution.<br>
ANSWER: He needs 375 ml of the 5% solution.<br>