The given expression is
[tex]0.00045-2.5\times10^{-5}[/tex]First, we express the first number in scientific notation. We have to make sure that both scientific notations have the same exponent.
[tex]45\times10^{-5}-2.5\times10^{-5}[/tex]Then, we subtract the coefficients.
[tex](45-2.5)\times10^{-5}=42.5\times10^{-5}=4.25\times10^{-4}[/tex]