Amazing Math Trick 234X234+1234-875=??? TRICK! TRICK! This trick makes you look like your brain is a mega fast calculator! Ask a friend to write down ANY three digit number such as 231 or 884. Ask them to multiply the number by x 7 x 11 x 13 ...but even if your friend has used a calculator, you will have written down the answer ages ago! It's fiendish!