Why do the first column and the top two elements of the second column refer to the original alloy while the bottom element of the second column refers to the modified alloy? Why do you multiply 0.2 by x? 20% = 0.2 is the ratio of copper to the total mass of the original alloy; therefore, it should be multiplied by the total mass of the original alloy. Similar point goes for 0.8(20-x). What is 0.2x+0.8(20-x)?
I am not sure if you were taught to make a table of this form. I would recommend denoting the mass of additional copper by x and writing what you can find with (or without) it: the mass of original copper, of copper in the modified alloy, total mass of the modified alloy, etc.