We let x = pounds of alloy that is 15% tin
The problem is modeled by the equation:
0.11(30) = 0.15x + 0.09(30-x)
The equation means that the amount of tin does not change. Also, the amount of alloy (total weight - weight of 15% alloy) is (30-x).
Solving for x:
3.3 = 0.15x +2.7 - 0.09x
.06x = 0.6
x = 10 lbs
30-10 = 20 lbs
10 lbs. of the 15% alloy and 20 lbs. of the 9% alloy