It is possible for a computer to pick up an erroneous signal that does not show up as an error on the screen. The error is called a silent paging error. A particular terminal is defective, and when using the system word processor, it introduces a silent paging error with probability .1. The word processor is used 20 times during a given week. a) Find the probability that no silent paging errors occur. b) Find the probability that at least one such error occurs. c) Would it be unusual for more than four such errors to occur? Explain, based on the probability involved.