math
posted by laura .
an average family of four uses roughly 1200 liters (about 300 gallons) of water per day (1 liter=1ooocm cubed) . how much depth would a lake lose per uear if it uniformly covered an area of 50 square kilometers and supplied a llocal town with a population of 40,000 people 
this is from a textbook, but I do know which one it is

Calculate the volume used per year, V, in cubic metres. Note that there are 1000 l. in 1 m³.
Calculate the surface area of the lake in square metres, A.
The decrease in depth, D, is V/A metres, assuming no river feeds the lake, and no change in volume due to evaporation and rain.
V=1200 (l.)/1000 (l/m³)*40000 (people) * 365 (days)
=1200/1000*40000*365 m³
S=50 (km²)=50*1000*1000 (m²)
Calculate V, S and D=V/S 
50,000,000 sq m
1.2x10,000 (10,000 families of 4 = 40,000 people)
12,000 m^3 per day * 365 days = 4,380,000 m ^3 per year
Depth = 4,380,000 m ^3 / 50,000,000
Depth lost = 0.0876 m^3