An alloy weighing 30 lbs. is 11% tin. the alloy was made by mixing a 15% tin alloy and a 9% tin alloy. how many pounds of each alloy were used to make the 11% alloy?

So.. we'll use the decimal format of their percentage, thus, 11% is just 11/100 or 0.11, 15% is 15/100 or 0.15 and so on

so hmm  

whatever the amounts of "x" and "y" are, we know, their sum is 30 lbs, since that's what the 11% mixture weights
thus    x + y = 30

now, the concentration of tin in each, must also add up to 3.3
thus 0.15x + 0.09y = 3.3

thus   

solve for "x", to see how much 15% alloy was used

what about "y"? well, y = 30 - x