According to a survey conducted by Harrah's Home Testing Institute, adults from 50% of U.S. households have gambled in a casino at some time in their life (American Demographics, October 1993). Assume that this percentage is true for the current population of adults from all U.S. households. A random sample of 400 U.S. households is selected.

(a) Using normal approximation to find the probability that at least 320 of the 400 selected households have gambled in a casino at some time in their life.

(b) Suppose a sample of 400 households shows that 320 have gambled in a casino at some time in their life. Is 320 an unusual number? Make your comments on this result.