How much depth would a lake lose per year if it uniformly covered an area of 80 square kilometers and supplied a local town with a population of 40,000 people? Consider only population uses, and neglect evaporation and so on.An average family of four uses roughly 1200 liters (about 300 gallons) of water per day. (1 L=1000cm^3)

a) What is the volume (in km3) of Avogadro's number of sand grains if each grain is a cube and has sides that are 1.5 mm long?   km3 (b) How many kilometers of beaches in length would this cover if the beach averages 111 m in width and 12.6 m in depth? Neglect air spaces between grains.  km

Consider a water reservior. The resrvoir graduallly loses water, because people insist on showering, washing clothes, etc.  The "use rate" R is the rate at which consumers use the water.   On April 13, between 1pm and 2pm, R is 5-gallons per second. How much water was drained from the reservoir between 1pm and 130pm?  At midnight, when everyone was asleep, a big industrial water user suddenly becomes active. Let A denote the amount of water taken from the reservoir (in gallons),  with A = 0 at midnight. It turns out that A=2t2 with t expressed in seconds. Given this information, calculate R, the use rate, at t = 10 s.