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)
