Annuity of teachers retired for disability.

The annuity of a teacher retiring under § 38-2021.04 shall be at least: (1) forty per centum of the average salary or; (2) the sum obtained under § 38-2021.05 after increasing his total service by the period elapsing between the date of separation and the date he attains the age of 60 years, whichever is the lesser.