The number of ways you can invite 3 of your friends on 5 consecutive days, exactly one friend a day, such that no friend is invited on more than two days is?
How should I proceed with this? What is the best method to solve this?
The answer given is 90.
You must invite two of them twice and one once. How many ways to choose the one you invite once? How many ways to choose the day you invite that friend? How many ways to distribute the other two?