You have two offices (President, Treasurer). Now you have to choose 2 people from a group of 5 how many ways can this can be done.
Spoiler:
There are five members of the Math Club. In how many ways can the positions of officers, a president, and a treasurer, be chosen? I thinked it's just 5 factorial. But the answer is 20. Which means they divided by 3 factorial... Why ?
See k-permutations.