79. Subject to sections 80 and 81, every individual who is a resident of Québec during a year shall pay for such year a contribution equal to 1.5% of his net income up to(a) $235 when his salary constitutes at least three quarters of his net income, and
(b) $375 in other cases, but his contribution shall then not exceed $235 plus the excess of his net income, excluding his salary, over an amount equal to one-quarter of his net income.