Occasionally, when I have a conversation with someone via instant message, I will want to type an exclamatory sentence ending with a digit number. For example, "I'll be back at 5!" In mathematics, a number with an exclamation point means to take the factorial of that number, so I usually put a space between the digit and the exclamation point, so that it will look proper to me, and not be ambiguous. (Even though most people don't even know what a factorial is.) Is there a proper way to express these kind of sentences in a clear way? I suppose something like "...five!" instead of "...5!" would be best, but what if I still want to use a digit instead of the word for the number? For that matter, is it improper to end a sentence with a digit? It's been a while since I've studied grammar, but I don't remember ever being taught this.