Tips for doing mental math:

If you want to find out what 10 percent of something is, move the decimal point to the left. So if you have the number 100, move the decimal point once to the left. 10% of 100 is 10.Some people would scoff at this idea, but it's a good idea to do most of your math in your head if you can. Because the more you do it the easier it gets. So if you're in a store walking around and figuring out what the tax is on stuff you see in there is a good exercise.

Also doing random math in your head on the bus is a good idea too. It's handy.  So in Nova Scotia(and maybe other places in Canada) the hst on a lot of things is 15%. So with the 10% trick it's easy to figure out what the tax is on something or a bunch of things.

So if I have something that costs $125. I move the decimal point to the left. I get 10% of the item. Then I split the 10% in half, then I add it to the result in my head then I have 15%

It's easy to screw up unless you know what the numbers are split in half. Once you're used to splitting the same number in half over and over after awhile you just know what it is split in half. Like half of 5 is 2.5 half of 50 is 25 etc.

You should do all of your math on your own in your head or on a piece of paper. Since relying on a calculator is just dumb. You should only do that if you're crunching huge numbers. I know it sounds like a boomer thing to do, but if you're too reliant on certain things it makes you mentally lazy.