What is the probability that the stock returns are less than 18.5%

According to Investment Digest ("Diversification and the RiskiReward Relationship", Winter 1994, 1-3l, the mean of the annual return for common stocks from 1926 to 1992 was 16.5%, and the standard deviation of the annual return was 19%. What is the probability that the stock returns are less than 18.5%