I <i>kind of</i> agree with you too. As with most things, it&#x27;s not black and white. It&#x27;s a spectrum. And the usefulness has shifted toward the &quot;less useful&quot; portion of the spectrum whereas in the past, mental math was far more useful and the ROI on being good at it was greater than it is in the present.<p>I&#x27;m not suggesting that you be totally braindead and not invest in learning the basics of mental arithmetic. I&#x27;m saying that going deeper than that initial basic investment isn&#x27;t as useful today and doesn&#x27;t make as much sense as in the past.<p>For example, I&#x27;d consider the 75^2 example in this blogpost very cool, but not worth for me to deliberately remember and so I&#x27;m not going to commit it to memory.