Sneha filled $\frac{1}{3}$ of the pages of her project file on the first day and $\frac{2}{9}$ of them on the second day. What fraction of the file is still empty?

A fraction a/b says: the whole is split into b equal parts, and we are talking about a of them. So 3/8 means: divide the whole into 8 equal parts, take 3 of them. The two equal parts is what makes it a fraction — three slices of an unevenly cut cake do not give you 3/8. The denominator must always be a positive whole number. The numerator can be 0, equal to the denominator (giving 1), greater than the denominator, or less than it.