Jessica is packing her bags for her vacation. she has 5 unique shirts, but only 3 fit in her bag. how many different groups of 3 shirts can she take?

The formula you can use for this involves factorials and is n!/r! (n-r) ! where n is the number of shirts and r is the different groups. So it would be 5!/3! (2) ! which can be simplified to 120/12 which is 10. If you don't know how to use factorials I can explain that as well.

... Or you could try it the long way and count out all the ways, but that's tedious.