Answer :
We start with the expression
$$
\frac{9}{10} - \frac{1}{4} = \frac{18}{20} - \frac{\square}{20} = \frac{13}{20}.
$$
Since the denominators are the same, we can equate the numerators. We have
$$
18 - \square = 13.
$$
Subtracting 13 from 18 gives
$$
\square = 18 - 13 = 5.
$$
Thus, the missing numerator is $5$, and the expression simplifies to
$$
\frac{9}{10} - \frac{1}{4} = \frac{18}{20} - \frac{5}{20} = \frac{13}{20}.
$$
$$
\frac{9}{10} - \frac{1}{4} = \frac{18}{20} - \frac{\square}{20} = \frac{13}{20}.
$$
Since the denominators are the same, we can equate the numerators. We have
$$
18 - \square = 13.
$$
Subtracting 13 from 18 gives
$$
\square = 18 - 13 = 5.
$$
Thus, the missing numerator is $5$, and the expression simplifies to
$$
\frac{9}{10} - \frac{1}{4} = \frac{18}{20} - \frac{5}{20} = \frac{13}{20}.
$$