How to Be a human calculator
Before we had the calculator and before we had the abacus we had the human brain to complete our math problems for us. Remember that? Well, it seems that few of us do, resorting to calculators on our phone for elementary subtraction problems and multiplication.
Strengthen your brain muscle by checking out this video on how to calculate fast than a calculator in less than 4 minutes. Because being good at fast mental math means you'll never get jipped.