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 lots of canned tuna fish. It isn't too expensive, and it is a good source of protein. Mary charges 79 cents a can all the time, but she has noticed that her sales have changed from time to time.
Explain why the demand for tuna fish changed in each of the following situations.
- When hamburger went up 20 cents a pound, Mary sold about 50 more cans of tuna fish each day. WHY?
- Mary sells about 100 more cans of tuna fish a day at the start of a month than at the end of a month. WHY?
- For a few weeks after there was an article in the newspaper about how healthful tuna fish is, Mary's sales increased 30 cans a day. WHY?
- There was a truck drivers' strike and food deliveries were interrupted. Mary sold 100 cans more a day. WHY?