#### Answer

$$3.0\times 10^{1}$$

#### Work Step by Step

Rearrange factors and multiply
$$(2.5\times 10^{-3})(1.2\times 10^{4})=(2.5\times 1.2)(10^{-3}\times 10^{4})=3\times 10^{1}$$
In scientific notation, the number of significant digits is the number of digits in the leading number between $1$ and $10$. The final answer should have the same number of significant digits as the least number of significant digits in any of the numbers involved in the multiplication.
$2.5\times 10^{-3}$ has two significant digits
$1.2\times 10^{4}$ has two significant digits
In this case, the number of significant digits in the final answer should be two:
$$3.0\times 10^{1}.$$