At the end of the accounting period the car will be worth less than when it was first bought therefore you will need to adjust it's value to reflect this.
As ruddy says depending on whether you are preparing monthly accounts or year end accounts will alter the amount of depreciation i.e. 11 or 12 months worth.
I would suggest that in your course question the answer would be:
New car cost £20,000
Less Depreciation @ 20% -£4,000
Net Book Value at year end £16,000