Question

The Central Intelligence Agency (CIA) has specialists who analyze the frequencies of letters of the alphabet in an attempt to decipher intercepted messages. In standard English text, the letter r is used at a rate of 7.58%.

a. Find the mean and standard deviation for the number of times the letter r will be found on atypical page of 3200 characters.

b. Give the interval of usual values for the number of r's appearing on a page of 3200 characters.
(Enter the answer as an interval in the form [a,b][a,b]. Round the endpoints to whole numbers that make sense in the context of the problem.)

c. In an intercepted message sent to a foreign country, a page of 3200 characters is found to have the letter r occurring 273 times. Is this unusual?

Answer 1