n represents the number of times a new number of persons have received the message and then start to pass it on starting with the original person.
When n = 1, only the first person has the message.
When n = 2, (4 minutes later) the first person and the three that he/she has told have the message.
When n = 3, (8 minutes later) the first four persons and the three that the the last group has told have the message.
Counting:
n = 4 ---> 12 minutes later
n = 5 ---> 16 minutes later
n = 6 ---> 20 minutes later
n = 7 ---> 24 minutes later
n = 8 ---> 28 minutes later
n = 9 ---> 32 minutes later
n = 10 ---> 36 minutes later
n = 11 ---> 40 minutes later
n = 12 ---> 44 minutes later
n = 13 ---> 48 minutes later
n = 14 ---> 52 minutes later
n = 15 ---> 56 minutes later
n = 16 ---> 60 minutes later
There are 16 "steps" -- don't forget to count the first one!