The examination must be held upon such subjects as in the opinion
of the board of examiners, with the approval of the Commission, the
general nature of the business of the office and the special nature
of the positions to be filled may require. In grading the competitors
due weight must be given to the efficiency with which the several
competitors shall have performed their duties in the office; but none
who shall fail to attain a minimum standard of 75 per cent in the
written examination shall be certified for promotion.