Question: how many 10 digit phone number combinations are there?
Answer: Well, this could turn out to be a simple permutation problem: you have ten number choices (0-9) for each digit of a phone number and repetitions are allowed. Technically, there could be as many as \begin{align*}10^{10} = 10,000,000,000\end{align*}, or 10 billion possible phone numbers in the U.S.