Page 598 bb q.18

cheerioswithmilk
Sun, 25 Nov 2012 08:29:36 +0000

First, nice username ;)

Think of the arrangement of the squares as positions. The spot furthest left is position one, followed by position two, and all the way to position five at the far right.

There are four possibilities for position 1: any of the squares can go there except the shaded one. Then, there are four possibilities for position 2: any of the four remaining squares can go there. Do this for each of the remaining positions, and you get 44321=72 arrangements.