Answer based on context:

The life expectancy at birth in Himachal Pradesh increased significantly from 52.6 years in the period from 1970-75 (above the national average of 49.7 years) to 72.0 years for the period 2011-15 (above the national average of 68.3 years). The infant mortality rate stood at 40 in 2010, and the crude birth rate has declined from 37.3 in 1971 to 16.9 in 2010, below the national average of 26.5 in 1998. The crude death rate was 6.9 in 2010. Himachal Pradeshs literacy rate has almost doubled between 1981 and 2011 (see table to right). The state is one of the List of Indian states and union territories by literacy rate with a literacy rate of 83.78% as of 2011.

How many years passed by as the Himachal Pradeshs literacy has almost doubled between 1981-2011?
30