Question: In another state, all license plates consist of from four to six symbols chosen from the 26 letters of the alphabet together with the ten digits 0-9.
a. How many license plates are possible if repetition of symbols is allowed?
b. How many license plates do not contain any repeated symbol?
c. How many license plates have at least one repeated symbol?
d. What is the probability that a license plate chosen at random has a repeated symbol?