How many milliliters of a 40% solution and a 12% solution are needed to make 420 ml of 25% solution?

You can use this type of chart and the allegation method to compute the answer to this. When filled in for this problem, it would look like this:
1. Number of parts for higher % solution = Desired % in final product - lower % solution: 25 - 12 = 13
2. Number of parts for lower % solution = Higher % solution - Desired % in final product: 40 - 25 = 15
3. Add the number of parts together to get a total number of parts: 15 + 13 = 28 total parts
4. Divide the final volume of the desired product by the total number of parts to get ml/part: 420/28 parts = 15 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:
15 ml/part x 13 parts = 195 ml of 40% solution
15 ml/part x 15 parts = 225 ml of 12% solution