Cards will appear for 2 seconds each. The first two cards will be a number 1-9. The next card with be an empty box. Before the card switches, press the number key that corresponds to the difference between the first two numbers. The next card will be a number 1-9, followed by another card with an empty box. You will need to press the number key that corresponds to the difference between the last two numbers given (NOT the previous difference). For example, if the first two cards are '4' then '6', the first difference will be '2'. Then, if the card is '9', the difference would be '3'. This pattern will continue through 5 differences.
Good Luck!