both are the same because the first line is going by ×2 so the missing number would be 22. The second line is going by +1 so 21+1=22. (Hope this will prove your friend wrong :3)
Yes they represent the same number because the first one (a) goes by to each line and the bottom one (b) moves one over each line. (Idk if that made sense but yes lol I’m bad at explications)
