Inverse Check Machine

About Inverse Check Machine

Inverse Check Machine hands the child a subtraction and an answer, and asks whether that answer survives being added back. If fifteen take away seven is claimed to be eight, then eight plus seven ought to return to fifteen — and if it does not, the answer was wrong. Checking your own work is a habit that almost no maths game teaches and almost every teacher wishes children had, and the inverse check is the first genuinely reliable version of it a child can use. It also quietly makes the case that addition and subtraction are two views of one relationship rather than two separate procedures, which is the same idea our Fact Family Triangle Flip game builds for younger children. About half of the answers the machine shows are deliberately wrong, and they are wrong in both directions, because a machine that is usually right teaches a child to wave everything through. There are three verdicts rather than two — it checks out, too small, too big — and that matters more than it appears: a straight yes or no can be guessed correctly half the time, whereas naming which way the error runs requires the child to actually do the addition and compare it against the number they started with. Numbers grow with the level and stay inside fifty. Every control is reachable by tap or keyboard and announced to screen readers, and a wrong verdict simply invites another look at the table. Free, no sign-up, no advertising to children, progress kept on the device.