I have a back yard that is 50 feet wide and 100 long. I want to build a cement block fence across the width of the yard and down one side of the yard. How many block will I need?

A cement block is 8"X8"x12".
Therefore you will need 150 blocks per course.
A course is 8" high.
The number of courses you will need is the height of the fence in inches divided by 8.
Therefore you will need 150xheigth (in inches)/8.

Silly question. Why is this in the sports forum, sub category fencing. This section is for the sport of fencing, not building a fence.