On 5.18, since you do not care exactly who was born on each day but only about how many people were born on each day of the week, you can proceed similarly as in Example 5.3, where the people are the cookies to buy and the days of the week at the available types of cookies.

For 5.20, we do care about who was born on each day, so one way to proceed is assign each of the students a day of the week making sure to not repeat days. How many available days does the first person have to choose from? The second? The third?

On 5.23a, to order a 3-topping pizza with no repeated toppings you just need to choose 3 ingredients out of the available ingredients. In how many different ways can you choose 3 ingredients if you do not care about the order in which you choose them?