The object of this quiz is to adjust each circle's radius so it contains the target population shown at the right but ONLY within Europe. When you click Start you will be shown a marker on a random populated Europe grid square (there are over 90K 10x10 km grid squares used for the data, courtesy of https://human-settlement.emergency.copernicus.eu/index.php). Also you will see an adjustable circle of radius 100 km to start. Adjust the circle's radius by dragging it until it contains the target number of people shown - this target will be between 5 and 100 million people. Then click submit. You are scored on how close your circle's population is to the target.
PLEASE NOTE: ALL of Turkey is included in counting population (not just European portion), and the portion of Russia west of 61.5 degrees E (just east of Chelyabinsk). Nothing outside Europe (such as Africa or middle east other than Turkey) is included toward the population count within a circle.
Quiz Takers 
the locations of the cities are very of, never in the center of the cities just vaguely in the right area.
Yeah the city centroids are based on the border of the whole city I believe.
These seem to be based on urban areas and some of the pins (Thisted for example) are way out in the countryside.
I think it’s just poor cordinates, A lot of the cities have pins right on a square border. Thisted is perfectly at 57N, Herning is perfectly at 9E. Hjørring and Brønderslev are perfectly at 10E, Helsingør is perfectly at 12º30’E, Vejle is perfectly at 9º30’E. etc.etc.
It’s centers of the boundaries I think based on the source (citypopulation), so yeah some aren’t going to appear right in the actual center of the populated area.
The pins on citypop are all accurate. I think you might be getting the municipal seat cordinates from the municipalities instead of the actual city. It would explain why larger cities like Frederikshavn and Helsingør are so far off while smaller cities and settlements in the municipality are more accurate. It’s actually quite bad, for example (on expert) it has Helsingør being in the square west of Hellebæk when it should be in the square east of it.
Interestingly the pins are accurate on the Denmark largest cities quiz.