![]() ![]() You get to choose from the same 10 choices again. The same thinking goes for the third digit of your password. Since you may use the same digit again, the number of choices for the second digit of our password will be 10 again! Thus, choosing two of the password digits so far, the permutations are 10 times 10, or 10 x 10 = 100 or 10 2. So for the first digit of your password, you have 10 choices. There are 10 digits in total to begin with. As you start using this new phone, at some point you will be asked to set up a password. Part 1: Permutations Permutations Where Repetition is Allowed Now let's take a closer look at these concepts. There may as well be water, sugar and coffee, it's still the same cup of coffee. It doesn't matter which order I add these ingredients are in. Like my cup of coffee is a combination of coffee, sugar and water. With Combinations on the other hand, the focus is on groups of elements where the order does not matter. ![]() If I change the order to 7917 instead, that would be a completely different year. That's number 1 followed by number 9, followed by number 7, followed by number 7. ![]() With Permutations, you focus on lists of elements where their order matters.įor example, I was born in 1977. The key difference between these two concepts is ordering. I'm going to introduce you to these two concepts side-by-side, so you can see how useful they are. Therefore the probability of winning the lottery is 1/13983816 = 0.000 000 071 5 (3sf), which is about a 1 in 14 million chance.Permutations and Combinations are super useful in so many applications – from Computer Programming to Probability Theory to Genetics. The number of ways of choosing 6 numbers from 49 is 49C 6 = 13 983 816. What is the probability of winning the National Lottery? You win if the 6 balls you pick match the six balls selected by the machine. In the National Lottery, 6 numbers are chosen from 49. The above facts can be used to help solve problems in probability. There are therefore 720 different ways of picking the top three goals. Since the order is important, it is the permutation formula which we use. In the Match of the Day’s goal of the month competition, you had to pick the top 3 goals out of 10. The number of ordered arrangements of r objects taken from n unlike objects is: How many different ways are there of selecting the three balls? There are 10 balls in a bag numbered from 1 to 10. The number of ways of selecting r objects from n unlike objects is: Therefore, the total number of ways is ½ (10-1)! = 181 440 How many different ways can they be seated?Īnti-clockwise and clockwise arrangements are the same. When clockwise and anti-clockwise arrangements are the same, the number of ways is ½ (n – 1)! The number of ways of arranging n unlike objects in a ring when clockwise and anticlockwise arrangements are different is (n – 1)! There are 3 S’s, 2 I’s and 3 T’s in this word, therefore, the number of ways of arranging the letters are: In how many ways can the letters in the word: STATISTICS be arranged? The number of ways of arranging n objects, of which p of one type are alike, q of a second type are alike, r of a third type are alike, etc is: The total number of possible arrangements is therefore 4 × 3 × 2 × 1 = 4! The third space can be filled by any of the 2 remaining letters and the final space must be filled by the one remaining letter. ![]() The second space can be filled by any of the remaining 3 letters. The first space can be filled by any one of the four letters. This is because there are four spaces to be filled: _, _, _, _ How many different ways can the letters P, Q, R, S be arranged? The number of ways of arranging n unlike objects in a line is n! (pronounced ‘n factorial’). This section covers permutations and combinations. ![]()
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