A boy found a bicycle lock for which the combination was unknown. The correct combination is a four-digit number. The digits are selected from 1, 2, 3, 5, 6, and 7.
a. What is the probability that he can open the lock at random if he knows that the combination is an odd number?
b. Wht is the probability that he can open the lock at random if he knows that the numbers in the combination cannot be repeated?