1. If you were tasked to buy just one donut the answer would be 5 as there are 5 to choose from
2. If you were tasked to buy two donuts the answer would be 5*5 as there are 5 to choose from for each choice.
3. To buy 6 donuts would be 56 different ways.
4. If you are counting the sequence of donuts, then a purchase of a plain donut followed by a sugar donut would be different than a sugar donut followed by plain donut.
5. However, the usual answer to a question on what's in a bag of donuts does not depend on the order purchased. So, to remove the combinations, the answer is 56/6!