What is x when 21 divided by x equals 7?

To find the answer, just divide the dividend by the quotient: 21 ÷ 7 = 3

To prove this, let's compute 21 divided by 3: $\frac{21}{3} = 7$

How does it work?

Asking what the value of x could be when 21 divided by x gives a result of 7, is equivalent to solving the following equation:

$\frac{21}{x} = 7$

Multiply both sides by x: $\frac{21 x}{x} = 7 x$
$\frac{21 x}{x}$ is equivalent to $\frac{x}{x} \times 21$ , which simplifies to $1 \times 21$ (or just $21$ ).
Thus, the equation becomes: $21 = 7 x$
To find x, divide both sides by 7: $\frac{21}{7} = \frac{7 x}{7}$
Which gives, after simplification: $3 = x$ (or $x = 3$ )

So, if the dividend is 21 and the quotient is 7, the divisor is 3.