What! Not more sums.
Sorry about that chief but they're such good fun I can't help myself.
This 'trick' just opens up like a flower as you do it so take your time to get it right, OK?
Ask a friend to select a number between 1 and 9 then;

• In your head, multiply that number by 9 (Say the selected number was 4 your answer will be 36)
• Multiply 12345679 by that number (note that there is no 8 in this number)
• Try to not look too surprised as the result unfolds.