Explain why there must be a mistake in each of the following statements:

(a) The probability that a student will get an \(A\) in a geology course is 0.3, and the probability that he or she will get either an \(A\) or a \(B\) is 0.27.

(b) A company is working on the construction of two shopping centers; the probability that the larger one will be completed on time is 0.35 and the probability that both will be completed on time is 0.42.