Answer
$6.3*10^5 \text{kg}$
Work Step by Step
Volume of lake in mL = $$100 \text{ square miles}*20 \text{ feet}= 5.57568*10^{10} \text{ ft}^3*\frac{28316.8 \text{ mL}}{1 \text{ ft}^3}=1.578854*10^{15} \text{ mL}$$
$$1.578854*10^{15} \text{ mL}*\frac{0.4 \mu\text{g Hg}}{1 \text{ mL}
}*\frac{1 \text{ kg Hg}}{10^9 \mu\text{g Hg}}=6.3*10^5 \text{kg}$$
