The number of years N(r) since two independently evolving languages split off from a common ancestral language is approximated by N(r) = -5000 ln r

The number of years​ N(r) since two independently evolving languages split off from a common ancestral language is approximated by

N(r) = -5000 ln r

where r is the proportion of the words from the ancestral language that are common to both languages now.

a. N(0.9) =