I think mental arithmetic is only necessary with reasonably small numbers. I see no problem with using calculators for problems like 999×999. but, I do think people should be able to do $70×7, to be able to figure out 30% off while shopping at a store, or 9×2, so they know to leave just under $18 on a $90 restaurant check. And, how to make change with simple subtraction. Practical math that comes up in daily life. However, most of my friends don’t even understand the math I mentioned, so I don’t think they are going to care to do the more advanced math you want people to do in their heads.