First choose a state, you’ll then be shown a US city from that state 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 100 largest cities in that state (or fewer if less than 100 cities).

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