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Question:

How many milliliters of a 30% solution and a 10% solution are needed to make 800 ml of 25% solution?

A 600 ml of 30% solution and 200 ml of 10% solution
Explaination

Use the allegation method and a chart like this. Chart filled in for this problem:
1. Number of parts for higher % solution = Desired % in final product - lower % solution: 25 - 10 = 15
2. Number of parts for lower % solution = Higher % solution - Desired % in final product: 30 - 25 = 5
3. Add the number of parts together to get a total number of parts: 15 + 5 = 20 total parts
4. Divide the final volume of the desired product by the total number of parts to get ml/part: 800/20 parts = 40 ml/part
5. Multiply the number of parts for each % solution by the ml/part to get the volume of each solution you need to add together to get the desired final product.
40 ml/part x 15 parts = 600 ml of 30% solution 40 ml/part x 5 parts = 200 ml of 10% solution