Question

When school closes, A school uses a phone tree to contact each student's family the principals' secretary calls 3 families, then each family calls 3 other families and so on. How many families will be notified during the 7th round of calls?

Answer 1

First round:

$1\to 3^1=3$

Second round:

$2\to 3^2=9$

Third round:

$3\to 3^3=27$

and so on,

Seventh round:

$7\to 3^7=2187$

$2187$ families will be notified during the 7th round of calls.