Chapter 5 - Section 5.2 - More Work with Exponents and Scientific Notation - Exercise Set - Page 268: 81

$8.0\times10^{-5}$

Using laws of exponents, the given expression simplifies to \begin{align*} & \dfrac{0.00064\times2000}{16,000} \\\\&= \dfrac{6.4\times10^{-4}\times2.0\times10^{3}}{1.6\times10^{4}} \\\\&= (6.4\times2.0\div1.6)(10^{-4}\times10^{3}\div10^{4}) \\\\&= 8.0\times10^{-4+3-4} \\\\&= 8.0\times10^{-5} .\end{align*}