A friend gave me this problem; it's been years since I've taken probability, and I'm really rusty. I'm curious how to solve it. Suppose you have a normal pdf with mean mu and stdev sigma that represents the distribution of test scores. What's the probability that two randomly selected students will have an average higher than k? So if I let X1 and X2 be the test scores of two randomly selected students, and if they have values of k1 and k2, then I want to know the probability that: k1 + k2 > 2k or P(X1 + X2 > 2k) I'm stumped. How does one approach a problem like that? Thanks!