Divide the diameter by 2 to get the radius. Then use the formula for the area of a circle (pi x radius2).
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Since the area of land is obviously not a perfect rectangle, you will need to know the angles that the various sides are set on in order to calculate precise acreage. However, if it's close to rectangular, then it will be approximately 3.7 acres.
That all depends how good the brakes were, how good the tyres were and the road conditions (amongst other considerations, for example if the wheels lock and a skid ensues or not - the best braking occurs when the wheels are just about to lock up). If the brakes had failed, it would depend upon all the the friction available (in the wheel bearings, between the tyres and the road, air drag, etc). If the crumple zones were being tested, the car would be fired towards a rather solid lump of concrete, so with a suitable lump of concrete, the car could be stopped in no distance; however the car can be guaranteed to be rather shorter than before it stopped, but it still will not have gone any further than the concrete lump. Finally, if the emergency stopping distances as described in the highway code were used, the stopping distance would be 80ft thinking distance + 802/20 ft stopping distance = 400ft ~= 122m. Perhaps you can give some more information, specifically the braking force that will be applied?