Question 15
Hạnh writes the fractions $\frac1{998},\frac2{997},\frac3{996},\frac4{995},\ldots,\frac{995}{1},\frac{996}{3},\frac{997}{2},\frac{998}{1}$ on the board. How many of these written fractions use exactly two distinct digits? Examples: $\frac9{990},\frac{333}{666},\frac{445}{544}$.