A local post office weighs outgoing mail and finds that the weights of first-class letters are approximately Normally distributed with a mean of 0.69 ounce and a standard deviation of 0.16 ounce.

a. Estimate the 60th percentile of first-class letter weights.

b. First-class letters weighing more than 1 ounce require extra postage. What proportion of first-class letters at this post office require extra postage?