<p>Ok - let’s start with Permutations</p>
<p>The factorial of a positive integer n, written n!, is the product of all positive integers less than or equal to n.</p>
<p>n!=1∙2∙3⋯n
0! is defined to be 1 So n! is defined for all nonnegative integers n.</p>
<p>A permutation is an arrangement. The number of permutations of n things taken r at a time is nPr = n!/(n-r)!. For example, the number of permutations of {1, 2, 3} taken 2 at a time is 3P2=3!/1!=6. </p>
<p>These permutations are 12, 21, 13, 31, 23, and 32. </p>
<p>On the SAT you do not need to know the permutation formula! Just use your calculator.</p>
<p>To compute 3P2, type 3 into your calculator, then in the MATH menu scroll over to Prb and select nPr (or press 2). Then type 2 and press Enter. You will get an answer of 6.</p>