First choose a country, you’ll then be shown a Europe 10K+ population city from that country and a population slider below it. Move the slider to the city’s population and click SUBMIT. You’ll get a score from 0 to 100 based on how accurate your guess was. Complete 20 random cities from among the 10K+ cities in that country (or fewer if less than 100 cities). Only countries with at least 25 10K+ cities are included.

If you click on the slider you can use the left and right arrows to move it more accurately.