Why You Forget After 100 Questions — and How Spaced Repetition Fixes It

In 1885 the psychologist Hermann Ebbinghaus found that within a day of learning something new we forget about 40% of it; after a week only about 20% remains. Without review, more questions won't help. It's why a question you can solve today can go blank on you in the exam.

The idea behind spaced repetition is simple: review a question just as you are about to forget it, and the memory is reinforced — and it fades more slowly next time. A reference schedule: • Review 1: the day you first solve it • Review 2: 3 days later • Review 3: 1 week later • Review 4: 2 weeks later • Each interval after that roughly doubles

DuckMath's mistake notebook is built on exactly this principle. Every question you get wrong enters the notebook, and the system schedules its return based on your forgetting curve — you don't flip through a paper notebook every day; the mistakes come back to test you. Once you answer a question correctly three times in a row it counts as mastered and stops appearing often. Your effort stays where it's needed most.

Most students revise by re-reading notes or worked solutions once more — the weakest way to revise there is. Real revision is active recall: cover the question, force yourself to reconstruct the steps, then check. Studies put the retention rate of active recall at about three times that of passive re-reading. DuckMath makes you write out every step rather than pick an option, precisely to build that habit.

Now go practise

Understanding the theory is not enough — practice is what makes it stick. DuckMath has a topic-organised, human-reviewed question bank waiting.