Why must old currency be taken out of circulation when new currency is made?

In what way did expansion and global trade during the 15th to 18th centuries affect the economies of european countries?

The soviet union and the united states believed in two very different economic systems. what were the names of each?

How did inca develop their road system? a) they used mortar to hold the stones together. b) they built suspension bridges over rivers. c) they mixed copper and stone together to make the blocks. d) they laid roads in the cities, not in the less densely populated deserts.