What is x when 57 divided by x equals 21?

To find the answer, just divide the dividend by the quotient: 57 ÷ 21 = 2.7142857142857

To prove this, let's compute 57 divided by 2.7142857142857: $\frac{57}{2.7142857142857} = 21$

How does it work?

Asking what the value of x could be when 57 divided by x gives a result of 21, is equivalent to solving the following equation:

$\frac{57}{x} = 21$

Multiply both sides by x: $\frac{57 x}{x} = 21 x$
$\frac{57 x}{x}$ is equivalent to $\frac{x}{x} \times 57$ , which simplifies to $1 \times 57$ (or just $57$ ).
Thus, the equation becomes: $57 = 21 x$
To find x, divide both sides by 21: $\frac{57}{21} = \frac{21 x}{21}$
Which gives, after simplification: $2.7142857142857 = x$ (or $x = 2.7142857142857$ )

So, if the dividend is 57 and the quotient is 21, the divisor is 2.7142857142857.