In the United States, the female-to-male earnings ratio was 0.77 in 2009; female full-time, year-round (FTYR) workers earned 77% as much as male FTYR workers. Womens earnings relative to mens fell from 1960 to 1980 (56.7-54.2%), rose rapidly from 1980 to 1990 (54.2-67.6%), leveled off from 1990 to 2000 (67.6-71.2%) and rose from 2000 to 2009 (71.2-77.0%). When the first Equal Pay Act of 1963 was passed in 1963, female full-time workers earned 48.9% as much as male full-time workers.

Based on the above article, answer a question. How many percentage points difference is there between the highest and lowest female-to-male earnings ratio between 1960 and 1980?
2.5