You accept you might make a silly mistake while doing the calculation in your head.  How do you know you won&#x27;t make a silly mistake when you do the calculation using a machine?<p>If you do both, or at least estimate the result, you can check the results.<p>I don&#x27;t think you&#x27;d make a silly mistake when using a calculator, but I think other people might.<p>I think (although I have nothing to support me) that most people cannot use percentages in any meaningful way.  If you say &quot;I have a tv that currently costs $300, and I&#x27;m going to give you a 10% discount. What will it cost after the discount?&quot; then they can show you the buttons they&#x27;d push to get that result.  But if you ask them a slightly different version &quot;I have a washing machine that currently costs $800. I&#x27;ve already given you a 10% discount. How much did it cost before the discount?&quot; then I think you&#x27;ll find a bunch of people who don&#x27;t know what buttons to push. (Or worse don&#x27;t know that they don&#x27;t know and who&#x27;ll get a close but incorrect answer.)<p>So, that&#x27;s not the kind of mental math trick talked about in the article, but calculators are tools and tools are most useful to people who know how to use them and most calculators and calculator apps don&#x27;t make it easy for people who don&#x27;t know math.