Answer :
When you are multiplying using scientific notations, you add the powers.
For example,
[tex]\begin{gathered} 3\times10^5\times2\times10^6=6\times10^{5+6} \\ =6\times10^{11} \end{gathered}[/tex]And,
[tex]\begin{gathered} 3\times10^5\times2\times10^{-6}=6\times10^{5+(-6)} \\ 6\times10^{-1} \end{gathered}[/tex]When you are dividing using the scientific notations, you should substract the powers.
For example,
[tex]\begin{gathered} \frac{4\times10^7}{2\times10^4}=2\times10^{7-4} \\ =2\times10^3 \end{gathered}[/tex]