As economists Milton Friedman and Anna J. Schwartz have noted, the decade from 1869 to 1879 saw a growth of 3 percent per year in money national product, an outstanding real national product growth of 6.8 percent per year in this period and a rise of 4.5 percent per year in real product per capita. Even the alleged "monetary contraction" never took place, the money supply increasing by 2.7 percent per year in this period. From 1873 through 1878, before another spurt of monetary expansion, the total supply of bank money rose from $1.964 billion to $2.221 billion, a rise of 13.1 percent or 2.6 percent per year. In short, a modest but definite rise, and scarcely a contraction. Although per-capita nominal income declined very gradually from 1873 to 1879, that decline was more than offset by a gradual increase over the course of the next 17 years.
Question: How many years did it take for the total supply of bank money to rise 13.1 percent?
Answer: 5