There are thing we must already know:
[tex]\begin{gathered} 1\text{kilogram}=1,000\text{gram} \\ 1\text{gram}=1,000milligram \end{gathered}[/tex]From this, we must know that:
[tex]1\text{kilogram}=1,000,000\text{milligram}[/tex]The scientis needs 0.25kilograms. From this we calculate:
[tex]0.25\text{kilograms}=0.25\times1000000milligrams=250,000milligrams[/tex]