These questions are making me confused: $\frac{2.10^{-7} - 0,4.10^{-6}}{10^{-8}} = ? $

Question

$$\frac{2.10^{-7} - 0,4.10^{-6}}{10^{-8}} = ? $$

These questions are making me confused because we're dealing with the terms like $10^x$. What are your professional tips?

My attempt:

$$\frac{2.10^{-7} - 4.10^{-7}}{10^{-8}} \tag{1} $$ $$\frac{ -8.10^{-7}}{10^{-8}} \tag{2} $$

And that's where I'm stuck.

Answer 1

From here

$$\frac{2\cdot 10^{-7} - 4\cdot 10^{-7}}{10^{-8}}=\frac{-2\cdot 10^{-7}}{10^{-8}}=-2\cdot 10^{-7}\cdot 10^{8}=-20$$

or as an alterntive

$$\frac{2\cdot 10^{-7} - 0.4\cdot 10^{-6}}{10^{-8}}=2\cdot 10^{-7}\cdot 10^{8} - 0.4\cdot 10^{-6}\cdot 10^{8}=20-40=-20$$

Answer 2

multplying numerator and denominator by $10^{8}$ we get $$2\cdot 10^{-7}\cdot 10^{8}-0.4\cdot 10^{-6}\cdot 10^{8}$$ and we get $$2\cdot 10-0.4\cdot 10^2=...$$

Answer 3

$(2)$ is incorrect.

From $(1)$, $\displaystyle \frac{2\cdot10^{-7} - 4\cdot10^{-7}}{10^{-8}}=\frac{(2-4)\cdot 10^{-7}}{10^{-8}}=\frac{-2\cdot 10^{-7}\cdot 10^8}{10^{-8}\cdot 10^8}=\frac{-2\cdot 10}{1}=-20$

Answer 4

$$\frac{2.10^{-7} - 0,4.10^{-6}}{10^{-8}} = 10^8 (2.10^{-7} - 0,4.10^{-6})=20-40=-20$$