Probability of any particular letter in a given text...

I'm doing a project for a class and I have to teach probability. We're going to play a game of Wheel of Fortune. I need to be able to show the class how to determine the probability of a particular letter occurring in the English language. Each student will be assigned a different letter, and will be given a short excerpt to read... about 300 or so words. They will have to count the number of times their letter appears and, from that, determine the probability of their letter. How do I find the probability?

You should divide the total number of times that a given letter appears by the length of the passage in letters.