§ 16-b. Computation of pensions in certain retirement funds. Notwithstanding the provisions of any other law, general or special, the members of every teachers' retirement system in all cities of the state shall, when retired pursuant to the law governing such system, receive a pension, if lawfully entitled thereto, based on the average annual compensation earnable as a teacher during the five years immediately preceding the date of his retirement, or the average annual compensation earnable as a teacher during any ten consecutive years of state service, said ten years to be selected by the member prior to date of retirement.