The answer actually is 4,245 BUT that is the correct way to do it because you take the smaller population away from the larger population in order to find their difference in sizes.
233 base five = 2 x 5^2 + 3 x 5 + 3 x 1 = 2 x 25 + 15 + 3 = 50 + 18 = 68 base 10
11000 base two = 1 x 2^4 + 1 x 2^3 = 16 + 8 = 24 base ten
43E base twelve = 4 x 12^2 + 3 x 12 + 11 x 1 = 4 x 144 + 36 + 11 = 576 + 47 = 623 base ten
Let x be your first alloy
2/5x+7/10(30-x)=1/2(30)
4x+210-7x=150
3x=60
x=20
So, you need 20 lbs of the first alloy, and 10 parts of the second to make 30 lbs of half iron and half silver alloy!