For the walls of the house John would use large planks of wood. Each plank needs 2 pieces of nails to be secured and he would use 16 planks. How many nails does John need for the house wall?
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John needs 2 nails for each plank. 
There are 16 planks. 

So, John needs 2 * 16 = 32 nails for the house wall.