An alloy of tin is 15% tin and weights 20 pounds. a second allow is 10% tin. how many pounds of the second alloy must be added to the first to get a 12% mixture? 30 lb 40 lb 60 lb

Let x represent the amount to be added. The total amount of tin will be
  15%·20 + 10%·x = 12%·(20+x)
  (15%-12%)·20 = (12%-10%)·x
  3%·20/2% = x
  30 = x

30 pounds of 10% tin must be added to get a 12% mixture.

The answer is: a = about 63.59ft2 for 15