Let the weight of 15% tin be x.
So the weight of the 8% tin will be: (40 - x), since the total weight is 40 lbs
Using the mixture formula:
w1*p1 + w2*p2 = w*p
Where w1 = weight of the first percentage, p1 = First Percentage
w2 = weight of the second percentage, p2 = Second Percentage
w = Final weight, p1 = Final Percentage
0.15*x + 0.08(40 - x) = 40*0.1
0.15x + 3.2 - 0.08x = 4
0.15x - 0.08x = 4 - 3.2
0.07x = 0.8
x = 0.8/0.07
x ≈ 11.429
Recall the 15% was made with x, which is ≈ 11.429
Recall the 8% was made with (40-x) ≈ (40 - 11.429) ≈ 28.571
So the 10% tin of the 40 lbs was made with ≈11.4 lbs of the 15% tin and ≈28.6 lbs of the 8% tin