Can you give the example of the last mental math calculation you did? I almost never do it and I still almost never did it even when I was in college taking higher math. Tip calculation is all I can think of.

I may chime in with another story. I‘m teaching operating systems at university and use these skills (for small numbers, and written multiplication/division for slightly larger ones) all the time. My general consensus after overseeing a few hundred students is that those that are unable to quickly do simple math in their head also struggle with the rest of the curriculum. Especially if they need a calculator for computing 57:2 everybody gets distracted and we have like 2 minutes idle time while everybody starts entering the equation. I’d rather spend my time teaching actual os stuff, it’s a shame enough that I have to teach how to decimal <-> binary <-> hexadecimal base conversion.

I work in big data/cloud stuff so I do estimates all the time to figure out what size resources I need/how long something will take. Less than 5 minutes ago I did mental math to see how long a job would take (I knew how long it would take for one read, and I had 2 days of reads over 3 data sources each with 8 data partitions => 48 reads each taking about 90 seconds => about 1.25 hours).

Tip is another thing. And I like to "min/max" grocery shopping so I typically do mental math when shopping to maximize things like grams of protein/$.

I do rough division all the time in my head, for instance figuring out how much each can of beer costs if a slab of 24 is $60. Or on the flip side, how much it's going to cost me to get a round of 4 beers at $8 each.

It's just a general life skill.

I can be claustrophobic..I don’t like being in a closed car..so when I am at traffic stops..I convert alphabets to numbers and numbers to alphabets of the license plates..and sometimes I add the numbers together to check if it’s a prime number. Calms me the fuck down.

in farming..construction..cooking..I use it all the time in my everyday life.