What is x when 51 divided by x equals 21?

To find the answer, just divide the dividend by the quotient: 51 ÷ 21 = 2.4285714285714

To prove this, let's compute 51 divided by 2.4285714285714: $\frac{51}{2.4285714285714} = 21$

How does it work?

Asking what the value of x could be when 51 divided by x gives a result of 21, is equivalent to solving the following equation:

$\frac{51}{x} = 21$

Multiply both sides by x: $\frac{51 x}{x} = 21 x$
$\frac{51 x}{x}$ is equivalent to $\frac{x}{x} \times 51$ , which simplifies to $1 \times 51$ (or just $51$ ).
Thus, the equation becomes: $51 = 21 x$
To find x, divide both sides by 21: $\frac{51}{21} = \frac{21 x}{21}$
Which gives, after simplification: $2.4285714285714 = x$ (or $x = 2.4285714285714$ )

So, if the dividend is 51 and the quotient is 21, the divisor is 2.4285714285714.