Suppose that in another country all automobile license plates consist of from four to six digits, any digit from 0 to 9 (one possible plate could be 1010 since it has four digits, another plate could be 10101 since it has five digits, and another plate could be 101010 since it has six digits) 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?