BeijingMath's Dynamic Difficulty Adjustment (DDA) system automatically generates engaging and challenging learning experience tailored to each individual. Using techniques drawn from machine learning and operational research, DDA system keeps students engaged and challenged by adjusting the level of difficulty (easy, medium, hard, and insane) for each question based on their past performances. Simply stated, DDA makes questions progressively harder with each correct answer. Therefore students are given questions at an appropriate level of difficulty, allowing them to work at a pace matched to their abilities.