If the picture of your great grandfather must be in the middle, how many ways can 7 pictures be hung?

720 ways to hang the pictures.

Explanation:
The answer can be found using the fundamental counting principle or 6!.

There are 7 pictures but the grandfather must be in the middle so it is the only possible choice. there are 6 other choices that must be made. the first picture has 6 possibilities, the 2nd has 5 possibilities, the 3rd has 4 possibilities, etc. giving us the possibilities can also be listed as:
6*5*4*3*2*1 or 6! =720.