An example of a miniature Sudoku puzzle is:
Notice that it consists of four rows, four columns, and four 2 x 2 boxes.
I was thinking of using the following wording for the instructions:
Fill each empty square with a number so that every row, every column, and every 2 x 2 box contains all the numbers 1, 2, 3 and 4 each exactly once.
I am uncertain whether my instructions should use the word CONTAINS or CONTAIN. Which is better and why? Any other help in improving the instructions would be appreciated.