You’re at the checkout screen after an online shopping spree, ready to enter your credit card number. You type it in and instantly see a red error message ...

Credit-card numbers, International Standard Book Numbers, bank routing numbers, Universal Product Codes, Cusip numbers used to identify stocks, municipal bonds and other financial instruments, plus ...

Adding larger numbers in your head is not always easy. If you need to use pen and paper, then the column method is a good way to add. It involves writing one number beneath the other, so that the ...