Multiplication of two numbers that differ by 2 (This trick only works if you have memorized or can quickly calculate the squares of numbers.When two numbers differ by 2, their product is always the square of the number in between these numbers minus 1. Let me explain this rule by taking examples 18*20 = 19^2 - 1 = 361 - 1 = 360 25*27 = 26^2 - 1 = 676 - 1 = 675