answer is about once a year
This is two or more equations with common variables, also known as a linear system. System Of Equations
B. <span>a shield of missiles over the U.S. designed to shoot down incoming nuclear missiles from the USSR</span>
I have this excerpt here about Norman Cantor's view on manorialism. I hope this has come to your help.
". . Of necessity, the manor was a self-sufficient economic unit in view of the overwhelming difficulties of transportation in the period. International trade was carried on only to serve the demands of the wealthy, and it was largely in the hands of aliens [different peoples]-Greeks, Jews, Moslems. Local society made almost no use of money. To the extent that local exchange was carried on, it was conducted by barter. The small amount of international trade precluded [ruled out] the need for gold coinage. The Carolingians minted only silver coins, which were all that was usually necessary when the smallest silver coin could buy a cow. When gold coins were needed, Byzantine and Moslem currency was used. . . ."
<span>Source: Norman F. Cantor, The Civilization of the Middle Ages, Harper Perennial</span>
