<p>“I think many people forget how to do divisions by hand.” - Wow, surprised by that. Admittedly I’ve forgotten how to derive a square root by hand. But even though I often use calculator, I still do remember how to divide. </p>
<p>Per calculating tips, I’ve seen many diners (men and women) pull out a calculator or laminated cheat-sheet. That boggles my mind. I assume all can determine 10%. Then if they can’t divide that by 2 and add those numbers together to get 15%… they could easily double for a generous tip 
Tipping is not an exact science - you usually round up or down anyway.</p>