Presently, the world population doubles after every (in $years$)

The $20^{th}$ century has witnessed a remarkable increase in the population of the world and especially in India. One major factor for this is that

According to the $2001$ census,the Indian population was:

World Population Day is observed on:

What are the probable factors that contributed to the population explosion in India?

What was the world's total population during the $2011$ census?