Find r in P(7, r)

Find r in P(7, r)

Recall the permutations formula:
7! / (7-r!) = 840.

We run 7! through our search engine and we get:
7! = 5040
5040 / (7 - r)! = 840

Cross multiply, and we get:
5040/840 = 7 - r!
6 = (7 - r)!

Since 6 = 3*2*! = 3!, we have;
3! = (7 - r)!
3 = 7 - r

To solve for r, we type this equation into our search engine and we get:
r = 4