Explain why there must be a mistake in each of the following statements
1. 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.
2. (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.