In function notation, where f is the function, f(x)=3x-10. Using f(x) means you can write things like f(0) which means: what is the value of the function when x=0? Think of f for flexibility, because functions have greater flexibility than variables like y.
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!