You would need to know the cost of the box of 20, then you would divide the total cost by the number of pencils in the box. This would give you the cost of one pencil - which is the unit cost.
Example:
Lets say the box of 20 pencils cost $10.
Divide the total cost by the number of pencils:
10 / 20 = 0.50 - This means each pencil cost 50 cents, so the unit cost would be 50 cents per pencil.