Adding second copper strip
Another way to explain it:
The reaction:
2 AgNO3 (aq) + Cu (s) ---> Cu(NO3)2 (aq) + 2 Ag (s)
can be written as:
AgNO3 (aq) + 1/2 Cu (s) ---> 1/2 Cu(NO3)2 (aq) + Ag (s)
so this 1/2 ratio is why we divide the count of gray copper by half.
1/2 Cu is represented by one gray copper, which means 2 gray copper agents represent 1 Cu atom.
so to know how many copper atoms are left when all we have are gray copper agents, we take the gray copper count and divide by 2.
Technical note: I set the slider on Silver Nitrate on purpose to jump in increments of 2 so that we do not end up with half an atom of copper left.