Stage 4 of the Rapid Mathematics track: seven trials in the checker's toolkit — inverse operations, divisibility rules, digital roots, and casting out nines and elevens. The modular tests that flag a wrong answer in a second, without redoing the work. No paper.
Each trial is locked until you clear the one before it. Every level scores out of 1000 — 600 for accuracy, 400 for speed — so speed alone can't clear the bar; you have to solve most problems, and quickly. Reach 650 to unlock the next.
You can reverse an operation, test divisibility by 3, 5, 9, and 11, and cast out nines and elevens to flag a wrong answer — including a sneaky transposition — in a single glance.