What is x when 21 divided by x equals 15?

To find the answer, just divide the dividend by the quotient: 21 ÷ 15 = 1.4

To prove this, let's compute 21 divided by 1.4: $\frac{21}{1.4} = 15$

How does it work?

Asking what the value of x could be when 21 divided by x gives a result of 15, is equivalent to solving the following equation:

$\frac{21}{x} = 15$

Multiply both sides by x: $\frac{21 x}{x} = 15 x$
$\frac{21 x}{x}$ is equivalent to $\frac{x}{x} \times 21$ , which simplifies to $1 \times 21$ (or just $21$ ).
Thus, the equation becomes: $21 = 15 x$
To find x, divide both sides by 15: $\frac{21}{15} = \frac{15 x}{15}$
Which gives, after simplification: $1.4 = x$ (or $x = 1.4$ )

So, if the dividend is 21 and the quotient is 15, the divisor is 1.4.