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.