Draw a grid on the whiteboard or overhead transparency:

You choose or a student chooses a mystery four-digit number. Each digit can be used only once.

The number is then marked using the following code.

Write this on the board or modelling book or use red, orange, and green transparent counters on the OHP instead of the code markings to cover the digits provided.

For example, suppose the correct number is 3 567 and the first student guess is 9 637, the table on a whiteboard would look like this:

The next guess might be 6 327. The 7 is definitely correct, the 6 and the 3 must be placed in new columns and the 2 is a possibility as it has not been used in the first try. Guesses and deductions continue until the correct answer is entered in the table.

The students can also play this game in pairs or small groups.