Answer
$$6.4\times 10^{5}$$
Work Step by Step
The number of digits in the number between $1$ and $10$ are the number of significant digits:
$1.6\times 10^{2}$ has $2$ significant digits
$4.0\times 10^{3}$ also has $2$ significant digits.
Multiply and rearrange factors:
$$(1.6\times 10^{2})(4.0\times 10^{3})=(1.6\times 4.0)\times (10^{2}\times 10^{3})=6.4\times 10^{5}.$$
