For grown-ups
RedPen is the Slate for older teenagers. Ellis, a first-year student, writes neat, fluent working across algebra, calculus, mechanics, stoichiometry and statistics. Each of the fifty derivations has one line where it first fails: a factor divided away (so a solution is lost), a square root with one sign, kilometres per hour treated as metres per second, averages of speeds instead of total distance over total time.
The reader finds that line, picks the corrected line from three, and names the kind of failure: an algebra slip, a wrong principle, a unit error or an unjustified step. Answers are cheap now, and checking is the skill. RedPen sits beside OverrideForge (when to rely on a tool) and BenchForge (how to test one).
Every error is written by people, not generated by a model. An exact maths engine checks every line, every fix and every wrong option, so nothing on screen is a trick. There is no Solve button and nothing to argue with.
Nothing needs an account or the internet. Progress stays in this browser.
What it never does
- No accounts, no ads, no tracking — anything kept stays on this device.
- No streaks, timers to beat, points to chase or leaderboards.
- No chat, and nothing anyone types is sent anywhere.