Could you help walk through the steps/formulas I'd have to use for this practice problem?

There is a new retail business opening locations in Texas. They are starting off with 4 clothing stores in Dallas and 8 stores in Austin. The distribution of revenue brought in in any week at each store is normally distributed with a mean of 3000 and standard deviation of 300. The sales at each store are independent of the sales made at the other stores.

a. What is the probability of both stores in Dallas selling at least $3500 in one week?

b. What is the probability that the average of the sales in Dallas is at least $3500 in one week?

c. What is the probability that the average sales revenue per store in one week in Dallas is more than $200 the average sales revenue per store in Austin?