Column Sub Exchange Cave

About Column Sub Exchange Cave

Column Sub Exchange Cave is the mirror of our column addition game and the harder of the pair. The grid holds the exchanged number at the top, the two numbers beneath it, and the answer along the bottom. Faced with 52 take away 27, the child cannot take seven from two, so a ten is exchanged for ten ones and 52 is rewritten as four tens and twelve ones before the subtraction is carried out. Writing that exchanged number down, rather than holding it in their head, is what prevents the single most common error in early subtraction: taking the smaller digit from the larger whichever way round they happen to sit, and confidently producing 35. A child who has physically rewritten the number has to confront that the top digit really was too small. Every problem in this game needs an exchange, for the same reason every problem in the addition game carries — a mixed set would let the step be skipped. The first two levels show the exchanged row so the child sees the rewrite before being asked to produce it; from the third level they do it themselves. The word used throughout is exchanging, which is the term used in British and Australian classrooms, where American material says borrowing or regrouping; children should meet the words they are taught. Every empty cell has a clue and a hint, grids are worked by tap or keyboard, cells are announced to screen readers, and wrong digits flash and can be replaced with nothing lost. Free, no sign-up, no advertising to children, progress on the device only.