Lisa sends an email to five friends

Lisa sends an email to five friends. The next day, each of Lisa's friends forwards the email to five more people. The process continues for a few days. What function represents the number of people who will receive the email on the nth day?

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​She starts by sending to 5 friends.  Each of those send to 5 more friends, and so on.

The first day’s recipients would be 5.  The second day’s would be 25.  The third day’s would be 125, and so on.

The function is 
C(n) = 5^n  {five to the n power, where n is the number of days}

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