Remark
If you just focus on the gray ones for a moment.
The series goes up 0 2 4 6
6 is in the fourth figure so you can count by figures.
f(n) = 2*(n - 1) So for the fourth figure, for example, you get f(4) = 2*(4 - 1) = 2 * 3 = 6 which is just what we see.
So far, the nth figure is f(n) = 2*(n - 1) But there is still 1 red one to account for.
Now we get f(n) = 2(n - 1) + 1 or 2n - 2 + 1 or 2n - 1.
Answer: f(n) = 2n - 1 <<<<< answer
