How it works
A sum appears with one number replaced by a box, and the box is not always at the
end. The child types what belongs there. The number range follows whichever of the six
levels is set, so this game grows with the child rather than staying at one size.
A wrong answer keeps the sum on screen. The first two misses say only "not yet";
from the third, the help shows a picture or the method, and then the answer.