To use permutation or combination formula?

A recurring question from many students who are preparing for CAT is this: When should one use the permutation formula and when should one use the combination formula? People have tried to answer this question in various ways, but some students still remain unsure. So we will give you a rule of thumb to follow in all permutation/combination questions: You never NEED to use the permutation formula! You can always use the combination formula quite conveniently. First let’s look at what these formulas do: Permutation: nPr = n!/(n-r)! Out of n items, select r and arrange them in r! ways. Combination: nCr = n!/[(n-r)!*r!] Out of n items, select r. So the only difference between the two formulas is that nCr has an additional r! in the denominator (that is the number of ways in which you can arrange r elements in a row). So you can very well use the combinations formula in place of the permutation formula like this: nPr = nCr * r! The nCr formula is far more versatile than nPr, so if the two formulas confuse you, just forget about nPr. Whenever you need to “select,” “pick,” or “choose” r things/people/letters… out of n, it’s straightaway nCr. What you do next depends on what the question asks of you. Do you need to arrange the r people in a row? Multiply by r!. Do you need to arrange them in a circle? Multiply by (r-1)!. Do you need to distribute them among m groups? Do that! You don’t need to think about whether it is a permutation problem or a combination problem at all. Let’s look at this concept more in depth with the use of a few examples. There are 8 teachers in a school of which 3 need to give a presentation each. In how many ways can the presenters be chosen? In this question, you simply have to choose 3 of the 8 teachers, and you know that you can do that in 8C3 ways. That is all that is required. 8C3 = 8*7*6/3*2*1 = 56 ways Not too bad, right? Let’s look at another question: There are 8 teachers in a school of which 3 need to give a presentation each. In how many ways can all three presentations be done? This question is a little different. You need to find the ways in which the presentations can be done. Here the presentations will be different if the same three teachers give presentations in different order. Say Teacher 1 presents, then Teacher 2 and finally Teacher 3 — this will be different from Teacher 2 presenting first, then Teacher 3 and finally Teacher 1. So, not only do we need to select the three teachers, but we also need to arrange them in an order. Select 3 teachers out of 8 in 8C3 ways and then arrange them in 3! ways: We get 8C3 * 3! = 56 * 6 = 336 ways Let’s try another one: Alex took a trip with his three best friends and there he clicked 7 photographs. He wants to put 3 of the 7 photographs on Facebook. How many groups of photographs are possible? For this problem, out of 7 photographs, we just have to select 3 to make a group. This can be done in 7C3 ways: 7C3 = 7*6*5/3*2*1 = 35 ways Here’s another variation: Alex took a trip with his three best friends and there he clicked 7 photographs. He wants to put 3 of the 7 photographs on Facebook, 1 each on the walls of his three best friends. In how many ways can he do that? Here, out of 7 photographs, we have to first select 3 photographs. This can be done in 7C3 ways. Thereafter, we need to put the photographs on the walls of his three chosen friends. In how many ways can he do that? Now there are three distinct spots in which he will put up the photographs, so basically, he needs to arrange the 3 photographs in 3 distinct spots, which that can be done in 3! ways: Total number of ways = 7C3 * 3! = (7*6*5/3*2*1) * 6= 35 * 6 = 210 ways Finally, our last problem: 12 athletes will run in a race. In how many ways can the gold, silver and bronze medals be awarded at the end of the race? We will start with selecting 3 of the 12 athletes who will win some position in the race. This can be done in 12C3 ways. But just selecting 3 athletes is not enough — they will be awarded 3 distinct medals of gold, silver, and bronze. Athlete 1 getting gold, Athlete 2 getting silver, and Athlete 3 getting bronze is not the same as Athlete 1 getting silver, Athlete 2 getting gold and Athlete 3 getting bronze. So, the three athletes need to be arranged in 3 distinct spots (first, second and third) in 3! ways: Total number of ways = 12C3 * 3! ways Note that some of the questions above were permutation questions and some were combination questions, but remember, we don’t need to worry about which is which. All we need to think about is how to solve the question, which is usually by starting with nCr and then doing any other required steps. Break the question down — select people and then arrange if required. This will help you get rid of the “permutation or combination” puzzle once and for all.