"maths" I think. I can still "add up", "multiply", "take away" and "divide". I have been able to do those things since early kittenhood - and I have kept on doing them. I don't need a calculator to do the "basic" things.
Yesterday someone put up a maths problem and asked who could solve it. It was an "A" level problem so I wouldn't have had much chance anyway. I only did the equivalent of "O" level. What is more I was not well taught - in fact I was very badly taught. It was only a bit of work on my part and that of my fellow students that actually got us through. Like physics the same teacher was one step ahead of us in the textbook. He wasn't qualified or trained to teach the subject.
I never discovered "calculus" and although remember (vaguely) the ideas behind trigonometry I have long since forgotten how to use "Logarithmic and other tables" or - as we called it - "Bogarithmic and other fables". I have never needed to use those things.
Someone needed to know what "14 x 22" was the other day. After a moment I said "308" and received a startled look because he was still hauling out his phone to use his calculator. But, that person is younger than I am. I suspect most people about my age who still have their wits about them can do that sort of thing "in their heads". The young ones use calculators.
I wonder though how many other things they fail to bother to remember now that we had to remember? I know less phone numbers than I used to know. The mobile numbers are longer and I don't use them often. I still try not to write a shopping list but make myself to remember it instead.
There are things I have always needed to read again - like cake recipes. I don't often cook cake. My mother and her mother could make cakes without looking at a recipe. They did it often.
I used to think it was the "frequency" that counted. Do something often enough and it becomes ingrained in your memory. Now I am less sure. If you always look the phone number up are you going to remember it?
When I was teaching and travelling to school each day I knew the train timetable so well I didn't need to look up train times even though I would sometimes need to be there early - or stay late. They changed the timetable some years ago and I still can't remember it because I don't use the train as often. If you are younger than I am then you don't need to remember. There is an "app" on your phone to tell you not just the timetable but where the train or the bus actually is at that moment.
I know the way people learn things is changing. The sort of things people are learning is also changing.
Yes, sometimes I regret not knowing more about maths but doesn't matter in the least if I can't do that problem. I am not planning on a career in astronomy or space technology.
What I don't regret is being able to remember the things I do remember. Does it matter? I think it does.