LESSON: Permutation Probabilities

Sometimes when dealing with permutation calculations, some of the objects may be repeated. That means that two (or more) objects are indistinguishable. When one object is indistinguishable from another that means we over-count the total number of possible permutations. Take a look at the following example so you can see what I mean.

Let's say you have 2 red marbles and 1 blue marble. How many ways can you arrange these marbles in a straight line? There are six ways you can do this! 

Notice though, that the red marbles are identical to each other, so even though there are six ways to order those three marbles, only three of those ways are distinguishable.

If asked to calculate a probability using permutations, we need to first make sure we are only counting the distinguishable permutations. Watch the next video to see examples of how to do this correctly.