Permutation

Permutation is similar to the counting principles discussed earlier. However, permutation questions involve options which are mutually exclusive.

Here is a sample ACT question involving Permutation:

Unlike with the security lock question covered above, once a plant is used in one display spot, it cannot be used in a different display spot. So instead of simply multiplying all of the possibilities together 6 * 6 * 6, now there is one fewer possibility each step of the way (since regardless of which plant is chosen, there will only be 5 plants to choose from for the second spot and 4 for the third).

The answers is therefore 6 * 5 * 4 = 120 or D.

Counting principle questions and permutation can intersect when there are limits introduced to the counting principle setup. For example, if the license plate example from above included the stipulation that no number or letter be repeated on a license plate, then the question would be similar to permutation. The number of options would reduce by 1 for each digit series (numbers or letters) that was continuously drawn from.