The number of permutations P(n,r) is the product of r numbers that starts with n, and each next number in the product is one less than the previous one.
120
Practice makes perfect
The number of permutations P(n,r) is the product of r numbers that starts with n, and each next number in the product is one less than the previous one.
P(n,r) = n * (n-1) * ⋯ * (n-(r-2)) * (n-(r-1))_(rnumbers)Let's use this fact to find the value of P( 5, 5). We will write the product of 5 numbers that starts with 5, and each next number in the product is one less than the previous one.
P( 5, 5) = 5 * 4 * 3 * 2 * 1_(5numbers)
Now, let's calculate this product.
P(5,5) = 120
