I was gonna post this as a comment but it'll be too long. As Mr. Wizard has shown you how to obtain the most popluar cities, here is how to plot them:

 Graphics[{{White, EdgeForm[Black],  CountryData[#, "FullPolygon"] &amp; /@ CountryData[]}, 
{PointSize[Large], Red,    Tooltip[Point[Reverse[CityData[#, "Coordinates"]]], 
          CityData[#, "Name"]] &amp; /@ largeCities}}]




Here, largeCities are the names of the cities without their populations. So you can do something like 

largeCities = Take[SortBy[numberofpeople, Last] // Reverse, 20][[All, 1]]


How do I find the most populous cities in the world?
I often do vector addition of Python lists.

Example: I have two lists like these:

a = [0.0, 1.0, 2.0]
b = [3.0, 4.0, 5.0]


I now want to add b to a to get the result a = [3.0, 5.0, 7.0].

Usually I end up doing like this:

a[0] += b[0]
a[1] += b[1]
a[2] += b[2]


Is there some efficient, standard way to do this with less typing?

UPDATE: It can be assumed that the lists are of length 3 and contain floats.
