Answer
$\approx3.06g$
Work Step by Step
Currently there is $100\cdot0.02=2g$ of bleach and the optimal is $5g$ of bleach, thus if we drain $x$ out of the 100g and replace it with bleach, the concentration of bleach will be: $\frac{2-0.02x+x}{100}=0.05\\2+0.98x=5\\0.98x=3\\x\approx3.06g$