I think most people who seem unable are just unwilling. I can do that in my head, but it's a lot of effort without practice. Someone less patient might put "37"

I think most people who seem unable are just unwilling. I can do that in my head, but it's a lot of effort without practice. Someone less patient might put "37" and "16" in their mind, start adding, and interpret the block they hit when trying to carry the 1 as "can't do it." It's hard to imagine not being able to do it if you're practiced, but it's as hard to do as anything else you never have to do when you never do it.