Add a hint feature.

[MHenderson]

If you don't know the answer you can click the help-me button and maybe it gives you one or other digit and you have to provide the other one.

[MHenderson]

Hint is the word I was looking for. Press hint once and four buttons appear, including the right answer. Press again and one wrong answer disappears. Press again and another one disappears. Answers without hints score 5 points, say, and using hints lowers the score for a right answer.