31. The duties payable by an operator on his annual profit, less the exemption, shall be:(a) 15 per cent of the profit not exceeding $3 000 000;
(b) 20 per cent of the profit exceeding $3 000 000 but not exceeding $10 000 000;
(c) 25 per cent of the profit exceeding $10 000 000 but not exceeding $20 000 000;
(d) 30 per cent of the profit exceeding $20 000 000.