Two stores sells landscaping rocks. At the first store rocks weight an average of 22 pounds, with a standard deviation of 2.5 pounds. At the second store the rocks are smaller, with a mean of 18 pounds and SD of 2 pounds. You select a rock at random from each store.

a. What are the mean and standard deviation of the difference in weights of the melons.

Mean of difference = 22-18=4

Standard deviation of difference = sqrt(2.5^2+2^2) = 3.202

Is this right?

b. Assume that the difference in weights has a normal distribution. What is the probability that the rock you get at the first store is heavier?

Any help with part b? Thanks.