Question

How many different groups of 3 students can be chosen from a class of 15 students?

Answer 1

5 because you can dived 15/3=5

Answer 2

"different groups" usually means a combination.

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I'd also like to point out, most scientific calculator has an [ ₙCᵣ ] button for Combinations as well as [ ₙPᵣ ] for Permutations.