Final answer:
The rest of the world used systems based on the Mercantilist economic system, focusing on the accumulation of precious metals, when the US adopted the decimal system for money.
Step-by-step explanation:
When the US adopted the decimal system for its currency, the rest of the world used systems rooted in the Mercantilist economic system. The key feature of mercantilism was the measuring of a nation's wealth in terms of gold and silver. Unlike the modern fiat money used today, which only has value based on governmental decree and public confidence, mercantilist economies focused on accumulating precious metals to control colonial and foreign markets through various taxes, tariffs, and other charges, with the aim of accumulating wealth and power.