A number when divided by 4 is reduced by 21. Find the number.

Let the required number be x.

When the number is divided by 4, the result is equal to 21 less than the original number.

$\therefore \dfrac{x}{4} = x - 21 \\[1em] \Rightarrow x = 4(x - 21) \\[1em] \Rightarrow x = 4x - 84 \\[1em] \Rightarrow 84 = 4x - x \quad \text{[Transposing +x to RHS]} \\[1em] \Rightarrow 84 = 3x \\[1em] \Rightarrow x = \dfrac{84}{3} \\[1em] \Rightarrow x = 28$

Hence, the required number is 28.