From eight different books, in how many ways can any four of them be arranged side by side on a bookshelf?
24
70
1,680

First we calculate how many ways you can choose four books from a set of eight.
We use the formula n! / [r! * (n-r)!]

8! / [4! * 4!]

= 8*7*6*5 / 4*3*2*1 = 70 ways

Then we have to calculate how many permutations can be made from 4 objects which equals 4*3*2*1 = 24

So, the TOTAL number of ways = 70 * 24 = 1,680

Polar coordinates is 8,9