Shelby, let's say that 2 people went to the rink. Then, it would cost the $100 plus $5(2) = $110 total. Since we just want an expression, we would write it as 100 + 5(2).
What about n people? Then, we have 100 + 5n. We write the term with the variable first, however, so we say 5n + 100. That is our answer.
Hope this helps!