Basically, the question is asking how many ways 20 people can occupy 3 positions. We'll use combination Combination is a method of selection of items from a collection, such that the order of selection does not matter.
So we have 20C3 ways where C denotes combination.
C(20,3)=20!/(3!(20â’3)!)= (20 * 19 * 18 * 17 *... 3* 2 * 1)/(3 * 2 * 1 * (17!))
17! = 17 * 16 * .... * 2 *1
So C(20,3)=20!/(3!(20â’3)!)= 1140
