A poultry farm has chickens and cows. When the caretaker of the poultry counted the heads of the stock in the farm, the number totalled up to 200. However, when the number of legs was counted, the number totalled up to 640. How many more cows were there in the farm?
Note: In the farm, each cow had 4 legs and each chicken had 2 legs.
- None of the above