LOGIC: If you count from 1 to 100, how many series of 7's and how many 7's will you pass on the way?

A friend posted this brain teasser and was wondering if someone here can explain it in Lamen's terms as if I was 7 years old. LOL

I know the how many 7's i pass between 1 to 100. Its 20.

But the series - has me stumped.

What is the answer & more importantly how do you write it?

Thanks in advance.