This app is "designed to adapt to a user's current ability, offering highly-tailored practice questions, tips for improving, and accurate performance assessments...
There are 100 difficulty levels for addition, subtraction, multiplication, and division."
Level advancement is based on the time it takes to type the right answer (not multiple choice). You advance independently for each operation.
The more you practice the higher your ranking in your region, your country and then the world.
(I am currently ranked 567 in the world, come and beat me - Siany representing Croatia :)