Chapter 4 - Polynomials - 4.2 Negative Exponents and Scientific Notations - 4.2 Exercise Set - Page 245: 113

$0.5\times10^{-5}$

Using the laws of exponents and the concepts of scientific notation, the given expression, $ (4.0\times10^{3})\div(8.0\times10^{8}) $, simplifies to \begin{array}{l} (4.0\div8.0)\times10^{3-8} \\\\= 0.5\times10^{-5} .\end{array}