Sides of the Mishkan

The northern and southern sides of the Mishkan had 20 planks each. The west side had 8 planks. The east side did not have any planks [Shemos (Exodus) 26:18-22]. Question: How many planks in total were used for the walls?

Answer:

20 x 2 = 40

40 +8 = 48 planks