I felt the same way, but I noticed professors I admired often doing arithmetic in their head, clearly because they enjoyed doing it. So since then I make an eff

I felt the same way, but I noticed professors I admired often doing arithmetic in their head, clearly because they enjoyed doing it. So since then I make an effort to mentally calculate. And you know what? It's a more interesting hobby than you'd think. Whatever technique you adopt (or grow, in my case), it's a good, concrete, occasionally useful exercise in breaking down a problem into smaller parts, and then combining those simple results into something else. Or maybe it's vanity. But I will say it's depressing to see all the cashiers (in the US) that can't seem to do any kind of math in their head, not even to minimize change (which seems totally ubiquitous in many other parts of the world; in fact, some cultures are aggressive about optimizing your change - China comes to mind).