This is an example of exponential growth, where the number of people who know about the band increases rapidly with each iteration. Assuming that Mia and her 3 friends are the only ones who know initially, the number of people who know after each iteration can be represented as follows:
First iteration: Mia and her 3 friends tell 3 people each, so now 1 + 3 x 3 = 10 people know.
Second iteration: Those 9 new people tell 3 people each, so now 10 + 9 x 3 = 37 people know.
Third iteration: Those 27 new people tell 3 people each, so now 37 + 27 x 3 = 118 people know.
Fourth iteration: Those 81 new people tell 3 people each, so now 118 + 81 x 3 = 361 people know.
And so on. As you can see, the number of people who know about the band grows very quickly, thanks to the exponential growth.