Answer:
Chain emails are emails with a message suggesting you will have good luck if you forward the email on to others. Suppose a student started a chain email by sending the message to 3 friends and asking those friends to each send the same email to 3 more friends exactly 1 day after receiving it.
Which day is the first day that the number of people receiving the email exceeds 100
f(n)=3(to the power of n)+1