context: In the early 1860s, Europe and the United States pursued completely different trade policies. The 1860s were a period of growing protectionism in the United States, while the European free trade phase lasted from 1860 to 1892. The tariff average rate on imports of manufactured goods was in 1875 from 40% to 50% in the United States against 9% to 12% in continental Europe at the height of free trade. Between 1850 and 1870 the annual growth rate of GNP per capita was 1.8%, 2.1% between 1870 and 1890 and 2% between 1890 and 1910; the best twenty years of economic growth were therefore those of the most protectionist period (between 1870 and 1890), while European countries were following a free trade policy.
question: How many year span the annual growth rate of GNP per capita was for 2.1%?
answer: 20