Q. 6 In a country license plates consists of four letters followed by three digits. Assuming that any letter or digit can be used (repetition is allowed), how many different license plates are...

Q. 6 In a country license plates consists of four letters followed by three digits. Assuming that any letter or digit can be used (repetition is allowed), how many different license plates are possible? What is the probability of license plate containing only one "D"? What is the probability of license plate containing only one "7"?