Mary runs the only grocery store in a 12 block area of a large city. Many of her customers are elderly and don't have much income. Mary sells a lot of canned tuna fish. It isn't too expensive and is a good source of protein. Mary charges 79 cents a can all the time, but she has noticed her sales have changed from time to time. Explain why the demand changed in each of the following situations.

Mary sells about 100 more cans a day at the start of the month then at the end of the month.