Permutation / Computation Guide

wisywigi
Thu, 01 Sep 2005 23:18:42 +0000

In order to be good at combinations/permutations, you must understand the logic behind them. When we learnt them in class, we started off drawing little boxes for each possible place. Then, when we understood what possibilities there were for each box, my teacher taught us nPr/nCr. To try doing those without understanding the basics is silly because you are much more likely to make mistakes then. So I’ll try explaining how we first learnt them but I might not be too clear as I don’t explain math very well. :-P

First, draw 5 little boxes from left to right. In the first box to the left, only 4 of the 5 cards can be placed there since the number 2 cannot be at the end. Next, skip over the 3 middle boxes and consider the last box. Since the number 2 cannot be there AND since you already placed one card in the box all the way to the left, there are only 3 possible cards that can go there. Now, go back to the second to left box. Since you already got rid of 2 cards (one at either end) AND since the number 2 CAN go there, there are 3 cards that can possibly be put there. As for the third box to the left, you only have 2 out of the 5 cards left and either can be put there. Finally, for the fourth box, you only have 1 card left! Now, simply multiple the possibilites and you get 43213 which equals 72. Sounds a bit long but once you get the hang of it, it is really simply to do.

I do not recommend using the nPr and nCr formulas because you can get confused easily if you haven’t practiced much with them. I hope this helped a bit. :-D