Try the following trick on a friend; then provide a mathematical justification to show why it works. Have your friend roll a die three times and record the results in order. Then give these instructions: Multiply the first number by 2. Add 5 to the result, and then multiply by 5. Add the Second number to this product. Multiply by 10 and add the third number. You then ask your friend to state the final result. From the number mentally subtract 250. The result will be a three-digit number whose digits represent the oringinal three numbers. (Instead of tossing three dice, you can merely have your friend think of a three-digit number.) The justification part is what I'm having trouble with.