Answer:
On the 40th and the 80th you will receive both a free beverage and a free appetizer. You can find this by simply finding the multiples of both number up to 100 (or a little more than 100) to find out on which dates you'd get bothe an appetizer and beverage for free. Here are the multiples of both, to prove this answer.
8: 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104.
10: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100.
Thus making 40 and 80 the answers. I hope this helps!