18.1. Let a pair of numbers represent the number of spots of the up-face of the first and second die, respectively. The probability of rolling a 7 is

The probability of rolling an 11 is

By Rule D, the probability of rolling a 7 or an 11 is

18.2. 45.6% is 2 standard deviations below the mean of 50%. The 68–95–99.7 rule tells us that 5% will be more than 2 standard deviations away from the mean. Half of 5%, or 2.5%, will be more than 2 standard deviations below the mean; that is, the probability that fewer than 45.6% say Yes is 0.025.