An alloy of tin is 15% tin and weighs 20 pounds. A second alloy is 10% tin. How many pounds of the second alloy must be added to the first to get a 12% mixture?

30 pounds of the second allow must be added to the first to get a 12% tin mixture.

20 * 15% = 3 pounds (of tin in the first alloy)

30 * 10% = 3 pounds (of tin in the second alloy)

3 + 3 = 6 pounds (tin); 30 + 20 = 50 pounds (alloy)

6 / 50 =.12 = 12% tin (in the mixture)