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　　The return on a portfolio is normally distributed with a mean return of 8 percent and a standard deviation of 18 percent. Which of the following is closest to the probability that the return on the portfolio will be between -27.3 percent and 37.7 percent?
　　A. 68.0%.
　　B. 81.5%.
　　C. 96.5%.
　　D. 92.5%.
　　Answer:D
　　Note that if you memorize the basic intervals for a normal distribution, you do not need a normal distribution table to answer this question. -27.3% represents a loss of -35.3% from the mean return (-27.3 – 8.0), which is (-35.3/18) = -1.96 standard deviations to the left of the mean. For a normal distribution, we know that approximately 95 percent of all observations lie with +/- 1.96 standard deviations of the mean, so the probability that the return is between -27.3% and 8.0% must be (95%/2) = 47.5%. A return of 37.7 percent represents a gain of (37.7 - 8.0) = 29.7% from the mean return, which is (29.7/18) = 1.65 standard deviations to the right of the mean. For a normal distribution, we know that approximately 90 percent of all observations lie with +/- 1.65 standard deviations of the mean, so the probability that the return is between 8.0% and 37.7% must be (90%/2) = 45%. Therefore the probability that the return is between -27.3 percent and 37.7 percent = (47.5% + 45%) = 92.5%.