Question 7
Hanh writes $\frac{1}{998} , \frac{2}{997} , \frac{3}{996} , \frac{4}{995} , \ldots , \frac{995}{4} , \frac{996}{3} , \frac{997}{2} , \frac{998}{1}$. How many fractions use exactly two different digits? Examples $\frac{9}{990} , \frac{333}{666} , \frac{445}{544}$.