A number, when divided by 4, is reduced by 21. What is the number?

Let the number be $x$.

According to the question:

When the number is divided by 4, it is reduced by 21.

So,

$x - \frac{x}{4} = 21$

$\frac{3x}{4} = 21$

$x = \frac{21 \times 4}{3} = 28$