Answer
$0.010$ kg
Work Step by Step
First we determine the mass of $100$ pills given that $1$ pill weighs $2.5\text{ g}$: $$100\cdot 2.5\text{ g}=250\text{ g}.$$ Now we determine the active mass in $250\text{ g}$ of pills: $$4\%\cdot 250\text{ g} = \frac{4}{100}\cdot250\text{ g} =10\text{ g}=0.010 \text{ kg}.$$
