Glad to see someone has as much trouble with arithmetic as I clearly do.
This does raise the question of what is a reasonable speed on a driveway. Inside our car park the limit is 5kph, a speed I and (it seems) other residents find difficult to maintain. A couple of steep access slopes make it almost impossible in anything faster than a farm tractor.
This website suggests 10 kph, which is not just reasonable but achievable. There’s no point in having limits that people can’t easily observe and maintain.