Picking melons

Two stores sell watermelons. At the first store, the melons weigh an average of 22 pounds, with a standard deviation of 2.5 pounds. At the second store, the melons are smaller, with a mean of 18 pounds and a stan-dard deviation of 2 pounds. You select a melon at random at each store.

a) What's the mean difference in weights of the melons?

b) What's the standard deviation of the difference in weights?

c) If a Normal model can be used to describe the difference in weights, what's the probability that the melon you got at the first store is heavier?