How many eight foot long 2 x 4's will you need to build a wall eight feet high and twelve feet long?

Okay, this requires a little simple math and some three dimensional thinking. For the fence (wall) planks alone, you will need 55 eight foot 2 x 4's. For a simple box frame you will need another 8.

55 + 8 = 63

The problem is, the fence (wall) will not be free standing. You'll need three twelve foot 4 x 4 posts (twelve foot long, because four feet of them will be set into the ground.