This question shows up often on standardized tests (SAT, ACT, GRE, etc) and can be solved in about two seconds 99.9% of the time if you memorize your square tables up to about 25 or so. Here's how to solve it: 1. Add 1 to the given product. 2. The answer is the square root of this number -/+ 1. Why it works: Any pair of consecutive odd or even numbers can be represented by X and X + 2, but also X - 1 and X + 1. We use this latter representation because it is in conjugate pair form, and the product will be X^2 - 1. So, all you are doing is reversing the process. Step 1 eliminates the -1 term. Taking the square root leaves you with X, a number which will be 1 greater and 1 less than the two numbers you are asked for in the original problem. Knowing this, you can memorize a list of common consecutive odd/even integer products, which will also equal the number in between^2 - 1: 11 x 13 = 12^2 - 1 = 143 12 x 14 = 13^2 - 1 = 168 13 x 15... read more