The rates of on-time flights for commercial jets are continuously tracked by the U.S. Department of Transportation. Recently, Southwest Air had the best rate with 80 % of its flights

arriving on time. A test is conducted by randomly selecting 11 Southwest flights and observing whether they arrive on time. (a) Find the probability that at least 7 flights arrive on time. (b) Would it be unusual for Southwest to have 4 flights arrive late?