5 Multiplication table
"Leaders aren't born they are made. And they are made just like anything else, through hard work. And that's the price we have to pay to achieve that goal, or any goal." - Vince Lombardi Multiplying by 5 is real easy because the result either ends in '5' or '0', so you know you have gone wrong if you get a number that doesn't end in '5' or '0'.