The following is a list of seven movies and their ratings:

1. March of the Penguins…………………………     G

2. National Treasure: Book of Secrets………     PG

3. Mamma Mia…………………………………         PG-13

4. Sex and the City…………………………              R

5. There Will Be Blood…………………………        R

6. The 40-Year-Old Virgin…………………………  R        

7. Showgirls…………………………                         NC-17

a. Find the modal film rating.

b. Find the median film rating.

c. Explain why it is inappropriate to calculate a mean film rating