Determine the information content of the message?

Question

Informatics

Determine the informational volume of the message: "There are 500 students in the school, they called to the director Petrov."
The answer must be: 9 bits. But I can not understand the topic and make the correct formula. In theory, they should give the power of the alphabet, but it is not here.

Answer 1

Hint:
1 bit = 2
2 bits = 4
3 = 8
4 = 16
5 = 32
6 = 64
7 = 128
8 = 256
9 = 512