I would need to use my hands or count out loud or do something after maybe two numbers if we are starting at 100.

Trying it out just now and the only way I can really effectively do it is by subbing 10 and adding three each time.

So I'm not in debt??? What a relief!

just as well, i always struggled with it.

And my teachers at school used to say "You wont always have a calculator on you"

Look at me now, i spend 7.5 hours a day paid to sit in front of an expensive calculator with two other expensive calculators in my pocket. after which i spend several hours on my own personal expensive calculator before going to bed and doom scrolling on one of the aforementioned.... expensive calculators.

Anything more than the most basic mental arithmetic at school was a waste of time that would have been better spent teaching me how to code. would have saved me teaching myself a few years later after working a soul crushing call centre job for 4 years (in which i used a not very expensive and dog shit slow calculator)