What I want is to slice the number off the front of the string so I need to know the length of the number. Adding an empty string to a number converts the number to a string. So (4 + "").length = 1, (13 + "").length = 2, and so on. A number by itself doesn't have a length property.

You're right, the problem only specifies "letters," which is open to interpretation. My solution passes all tests, so perhaps the author did not have foreign letters in mind.