[tex]0.\underbrace{00028}_{\rightarrow5places}=28\times10^{-5}\\\\0.00028:(7\times10^{-4})=\dfrac{28\times10^{-5}}{7\times10^{-4}}=\dfrac{28}{7}\times\dfrac{10^{-5}}{10^{-4}}\\\\=4\times10^{-5-(-4)}=4\times10^{-5+4}=\huge\boxed{4\times10^{-1}}\leftarrow\boxed{b.}\\\\used\ \dfrac{a^n}{a^m}=a^{n-m}[/tex]
