48 kg of 25% alloy should be used;
72 kg of 65% alloy should be used.
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I like to use a little X-diagram to work mixture problems. The numbers on the left are the contributions made by each constituent. The number in the middle of the X is the target value for the mix. The numbers in the center column are the differences along the diagonals. The numbers in the right column are in the same proportion but scaled to add to the total you need for this problem. (The scale factor is 3.)
