There are 18 ways to choose the first one; 17 ways to choose the second one, and 16 ways to choose the 3rd one. The same 3 students can be chosen in any of 3! = 6 different orders, so the product 18·17·16 must be divided by 6 to get the number of possible combinations in which order doesn't matter.
20:5 The total number of jelly beans is 20, and there are 5 orange jelly beans. This is a total to part ratio because it’s comparing the entirety to a part.