Question

A severe thunderstorm dumped 2.0 in of rain in 30 min on a town of area 22 km2. what mass of water fell on the town? one cubic meter of water has a mass of 103 kg.

Answer 1

For this case, the first thing you should know is that density is defined as the quotient between mass and volume.
 d = m / v
 We have then for this case that the density is:
 d = 103 Kg / m ^ 3
 The volume is given by:
 V = A * d
 Where
 A: area of the city
 d: height of the rain
 Also, for calculation purposes, you must know the following measure:
 1Km = 1000m
 1 inch = 0.0254 meters
 Calculating the volume we have:
 V = (22 * (1000/1) ^ 2) * (2 * (0.0254 / 1)) = 1117600 m ^ 3
 Then, calculating the mass we have:
 m = d * v
 m = (103) * (1117600) = 115112800 Kg
 Equivalently
 m = 1.15 * 10 ^ 8 Kg
 answer
 mass of water fell on the town was 1.15 * 10 ^ 8 Kg

Answer 2

We want to convert everything to meters so we can calculate cubic meter volume and use the given mass information.  
2 inches = 5.08 centimeters = .0508 meters.  
22 square kilometers = 22,000,000 sq meters.  
The product of these gives the number of cubic meters of water that rained. .0508 * 22,000,000 = 1,117,600 cubic meters.  
Multiply by the mass to arrive at final answer. 1,117,600 m^3 * 103kg/m^3 = 115,112,800 kg  
115,112,800 kg of water fell.