What is x when 39 divided by x equals 21?

To find the answer, just divide the dividend by the quotient: 39 ÷ 21 = 1.8571428571429

To prove this, let's compute 39 divided by 1.8571428571429: $\frac{39}{1.8571428571429} = 21$

How does it work?

Asking what the value of x could be when 39 divided by x gives a result of 21, is equivalent to solving the following equation:

$\frac{39}{x} = 21$

Multiply both sides by x: $\frac{39 x}{x} = 21 x$
$\frac{39 x}{x}$ is equivalent to $\frac{x}{x} \times 39$ , which simplifies to $1 \times 39$ (or just $39$ ).
Thus, the equation becomes: $39 = 21 x$
To find x, divide both sides by 21: $\frac{39}{21} = \frac{21 x}{21}$
Which gives, after simplification: $1.8571428571429 = x$ (or $x = 1.8571428571429$ )

So, if the dividend is 39 and the quotient is 21, the divisor is 1.8571428571429.