Answer
$0.00000002$
Work Step by Step
RECALL:
(i) Scientific notation has the form $a(10^n)$, where $1 \le a \lt 10$ and where $n$ is an integer.
(ii) $a^m \cdot a^n = a^{m+n}$
(iii) $\frac{a^{m}}{a^{n}}=a^{m-n}$
Write each number in scientific notation to obtain:
$=\dfrac{7.8(10^{-4})}{3.9(10^{4})}$
Divide $7.8$ and $3.9$ together, and then divide the powers of 10 together to obtain:
$=\dfrac{7.8}{3.9} \cdot \dfrac{10^{-4}}{10^{4}}
\\=2 \cdot 10^{-4-4}
\\=2 \cdot 10^{-8}
\\=0.00000002$
