Answer
8.76 cm/year
Work Step by Step
First and foremost, we find the volume of water used by a single person per day:
$$\frac{1200l}{4 people}=300l/person/day$$
Next, we find the volume of water used by the whole town in a year:
$$(40000people)(\frac{300l}{1person})(\frac{365 days}{1 year})(\frac{1000cm^3}{1l})(\frac{1km}{100000cm})^{3}$$
$$=4.38\times 10^{-3}km^3/year$$
Finally, we find the depth of water lost per year by dividing the volume by the area:
$$d=\frac{V}{A}=\frac{4.38\times 10^{-3}km^3/year}{50km^2}=(\frac{8.76\times 10^{-5}km}{1year})(\frac{100000cm}{1km})=8.76cm/year$$
