What it teaches
Recall rather than working out. A duel is won by the child who already knows that 7 × 8 is 56, not by the one who can get there in fifteen seconds, and that is a different skill from everything else on this site.
The four wrong answers are the mistakes children really make: one out, ten out, the digits the other way round, and the answer to the other operation. A race against four random numbers would be a test of eyesight.
How it works
The tablet lies flat and a child sits at each end. The top half of the screen is turned through 180 degrees, so each of them reads their own half the right way up.
The same question appears on both halves. First correct answer takes the point, and first to five wins. A wrong answer costs that player the rest of the question but never a point — taking points off a child who is already losing ends the game in tears.
Four levels: adding and taking away inside twenty, then inside a hundred, then the times tables, then all four operations at once.
What to look for
Nothing from this game is recorded. The other player is usually an older sibling or a parent, and counting their answers as the child's progress would spoil the one number in the Parents Area that is meant to mean something.
Play it yourself against the child and pick a level that makes it close. A duel that is not close is over in a minute and never asked for again.