Quickfire Sums — timed mental-math rounds with streaks and personal bests

Pick a topic and a level and race the clock: ten items on their own timers or a 60-second sprint, answered from the keyboard and marked instantly, with streak multipliers, a speed bonus and personal bests per topic. 1,176 checked items over ten topics (times tables, percentages, fractions, negatives, metric units and more) are free before sign-in. A coach writes fresh item sets for any topic at levels 1-5 - every item re-computed by your browser and rejected if wrong - and debriefs each miss with the slip, the rule, a memory hook and a worked line. Derived from @jamesrochabrun/math-teacher (jamesrochabrun/skills, MIT).