From Alma 11:4-19

The Book of Mormon money system is based on a 1-2-4-7 ratio
This is the most efficient money system, mathematically available and is used anciently only by the most sophisticated and advanced civilizations.
The United States uses a 1-5-10-25-50-100 system. This is a poor and inefficient system. Were the U.S. to use a 1-2-4-7 system, they would have, perhaps:

A 10 cent piece = 1
A 20 cent piece = 2
A 40 cent piece = 4
A 70 cent piece = 7