What is x when 81 divided by x equals 4?

To find the answer, just divide the dividend by the quotient: 81 ÷ 4 = 20.25

To prove this, let's compute 81 divided by 20.25: $\frac{81}{20.25} = 4$

How does it work?

Asking what the value of x could be when 81 divided by x gives a result of 4, is equivalent to solving the following equation:

$\frac{81}{x} = 4$

Multiply both sides by x: $\frac{81 x}{x} = 4 x$
$\frac{81 x}{x}$ is equivalent to $\frac{x}{x} \times 81$ , which simplifies to $1 \times 81$ (or just $81$ ).
Thus, the equation becomes: $81 = 4 x$
To find x, divide both sides by 4: $\frac{81}{4} = \frac{4 x}{4}$
Which gives, after simplification: $20.25 = x$ (or $x = 20.25$ )

So, if the dividend is 81 and the quotient is 4, the divisor is 20.25.