$8.4 \\times 10^{25}$ is the product of $2.1 \\times 10^{25}$ and what number?

Explanation:

Step1: Define unknown number

Let the unknown number be $x$. Set up equation: $$8.4 \times 10^{25} = (2.1 \times 10^{25}) \times x$$

Step2: Isolate the unknown $x$

Rearrange to solve for $x$: $$x = \frac{8.4 \times 10^{25}}{2.1 \times 10^{25}}$$

Step3: Simplify the expression

Divide coefficients and cancel $10^{25}$: $$x = \frac{8.4}{2.1} = 4$$

Answer:

$4$