I want to examine this question from a different angle.
What kind of person would take a step backward for every 2 steps already taken?
Perhaps we’re talking about a child, in which case the math will anyways get confused somewhere between 11-90.
Or maybe this is a severe case of OCD. I would recommend immediate hospitalization instead of trying to count his/her steps.
There’s also the chance this question has ADD/ADHD roots. Here we will never really find an answer, because they sold out in the bookstore.
All in all, I think this question needs to be rephrased to something more practical, so philosophers like me don’t notice it.