What is the probability that a seven-digit phone number contains the number 7?

Best way to answer this question is by figuring out the probability of a phone number not containing the number 7. There are 9 other numbers that are not a 7, so odds of each digit not being a 7 is 90%. For a 7-digit number, calculation would be 0.9*0.9*0.9*0.9*0.9*0.9*0.9 or 0.9^7. This equals 47.8%, which means the odds of a phone number containing at least one 7 is 100% - 47.8%, which is 52.2%. But phone numbers do not begin with 0 or 1, so if we account for that, the odds of the first digit not being a 7 is 7/8, not 9/10. We replace the first 0.9 with 0.875, and the result is 53.5%. There are other scenarios we can take out, like numbers that begin with 555, but I'll just leave it at that.