School days.

Anyone lucky enough to have had a primary school education prior to 1970 had the best possible start in life. No calculators, no extra subjects to cram into the curriculum at the expense of the basics.
The teachers had time to teach you how to read. You proved that could read by standing up in class and reading from a book. They had time to teach you how to spell. Our fourth year teacher had spelling competitions that were fiercely fought. I recall as a ten year old standing up in class to spell "encyclopaedia". It was competitive. It was fun.
They taught us arithmetic. We had to learn our tables, up to the twelve times tables. By heart. My dad said I had it easy. There was even less pressure on the curriculum in the 1930s, so he had to learn up to the twenty times table. We were taught fractions. We were taught mental arithmetic. Calculators hadn't been invented.
My headmaster at Penpol Primary School in Hayle was a Mr Mitchell. He was a grown up so didn't have a first name. He taught the fifth year, and got us prepared for the eleven plus exam. Out of a class of about thirty, and without classroom assistants, he enabled five of us to pass the eleven plus and go the grammar school. Competition for places was tough. There were less than 200 places for the whole of West Cornwall. Selection was on merit, on how well you did in your exam. You got into grammar school because you were the best.
All through that final year at primary school his motto was "speed and accuracy". But accuracy first, because without accuracy, speed is useless.

I read recently that GCSE examiners are told to mark as correct any mathematics answer that has the right numbers but with the decimal point in the wrong place.
I also read recently about a locum doctor who prescribed ten times the dose of a drug to a patient who subsequently died.
And they don't make the connection.

Before February 1971, the currency in use consisted of pounds, shillings and pence. Twelve pence to the shilling, twenty shillings to the pound, two hundred and forty pennies to the pound.
When I started work at the bank, all staff were expected to be numerate, that is to say, they could add up and take away. Each cashier was responsible for the contents of his till, which he would balance each day. He/she had the use of a tally roll calculator for adding up the "meat ticket" cash slips, but we prided ourselves on our ability to do arithmetic, and to do as much as possible without recourse to machinery.

I left the bank in 1970 and for a few months worked as an invoice clerk for a frozen foods firm. The telesales girls would ring their customers and write the order on an itemised invoice. The Invoice clerks would price the individual lines, add up the invoice and deduct the commission to leave the total at the bottom. There were four of us, working to a deadline, and we'd have hundreds to tally up during the course of a day. We were employed on the basis of our ability to add up. And we were good.
Altogether now-
Take 3 3/4% discount off an invoice for £17-6-4d.

No I can't do it now either. But I could back then. Without recourse to paper and pencil.

Our branch of the bank was busy. We had several large local firms, plus many local shops and department stores. At the close of business, we'd have a pile of cheques a foot high that had to be sent away to the clearing house. Each morning we'd receive a huge pile of cheques that had to be filed into each customer's file. This was called sortaway. Then we'd compile the statements, taking the cheques and paying in slips and arranging them into the order they appeared on the statement. and woe betide if a cheque was mis-sorted. It had to be found.
By the time this was complete, there was a pile of today's work to be done. There was always a deadline to hit. It was pressure. It was fun. From 3.30 when the branch closed the doors until 5.00 it was a rush to balance the tills (to the penny), transfer the cash to the vaults, process all the paying in slips and cheques, punch the data, and bag up the cheques for clearing.
It was fun if you could keep up.
I couldn't.
I used to watch the more experienced staff as they operated the various machines. Their fingers flew over the keys and they never seemed to look at what they were doing. And their work was almost always correct to the penny.
I had to be taught how to do that. So they sent me away for a week to learn. It's a skill that never leaves you. In recent years I've worked from time to time in data entry. I easily passed every assessment, both for speed and for accuracy.
Speed and accuracy. But accuracy first.