29. Before 1933, the United States issued gold coins for general circulation, the highest denomination being $20. (The U.S. mint still produces gold coins but for collecting and investment purposes.) Suppose that you want to make gold coins available again, with the value of the coin the same as the value of the gold in it. Such a coin would be round and have a thickness of 1 mm (that is half the thickness of current $1 coins and thinner than a dime). Gold has a density of ; and the volume of a cylinder is , where is the radius and is the height (thickness). At a price of $1200 per ounce, what would be the diameter of a $20 gold coin?