Computing Probability

Suppose that when the pH of a certain chemical compound is 5.00, the pH measured by a randomly selected beginning chemistry student is a random variable with mean 5.00 and standard deviation 0.2. A large bath of the compound is subdivided and a sample given to each student in a morning lab and each student in an afternoon lab. Let X = the average pH as determined by the morning students and Y = the average pH as determined by the afternoon students.

a.) If pH is a normal variable and there are 25 students in each lab, compute the probability P(-0.1 < X-Y < 0.1)
Hint: X-Y is a linear combination of normal variables, so is normally distributed with mean E(X-Y) = E(X)-E(Y) and variance V(X-Y) = V(X)+V(Y)

b.) If there are 36 students in each lab, but pH determinations are not assumed normal, calculate (approximately) P(-0.1 < X-Y < 0.1)

*X and Y are X bar and Y bar. I just cannot place bars above letter so that's the best I can do

I don't really want the answer given to me, but I honestly don't really understand the data that they are giving me because of how it is said. So perhaps if someone could break that down and give me a few of the basic overall steps that I need to do that would help me a ton!