A certain four-cylinder combination lock has 50 numbers on it. To open it, you turn to a number on the first cylinder, then to a second number on the second cylinder, and then to a third number on the third cylinder and so on until a four-number lock combination has been effected. Repetitions are allowed, and any of the 50 numbers can be used at each step to form the combination. (a) How many different lock combinations are there? (b) The probability that the correct lock combination is guessed on the first try