About 80% of finnish workers are members of a trade union. The number of labor disputes in Finland from 2000 to 2011 can be modeled by
f(x) = -72 + 73.6 ln x 10 <= x <=21
where x represents the number of years since 1990, and f(x) represents the number of labor disputes annually. Determine the average value of f on the interval [11,21] and interpret.