It might seem an unnecessary skill to some, but being able to do certain math problems in your head can come in quite handy.
For example, say you are at the grocery store and only have a certain amount of money, and don’t have a calculator with you; how do you know when you’ve reached your limit? Simple, you add things up in your head as you go along.
Or how do you know when you buy something with cash that you’re getting the right change back? Well, if you calculate how much change you should get back in your head before you hand over your cash, then you’ll know as soon as they count out into your hand.
These examples might seem simple and not that big of a deal, but being able to add how much things will cost in your head is a way to be someone who is able to do things like that; it’s a sign of maturity, and brains, and a way to show other people that you know what’s going on and are not someone to be trifled with.
At any rate, the way that you learn to do things like add up groceries in your head, is by practicing. Or in other words, you have to teach your brain to allow you to do it. Don’t start at the grocery store, at least if you have long list, try adding things at home first. But the key is first learning how to round off numbers first, if you don’t already know how. For example if you are going to buy a loaf of bread for a buck twenty nine, and gallon of milk for three fifty seven, you first round the bread to a buck thirty, and the milk to three fifty. Then all you have to compute is $1.30 + $3.50. It’s four eighty. Simple addition. Of course your estimates can cause you to be low on your count, but the more things you buy, the less rounding errors matter because some will be a little high and some a little how and eventually they pretty much cancel each other out.
So, that’s how you learn to add stuff up in your head.
As for estimating how much you should get back in change, that is something you just have to practice. Do it at home though because it takes awhile to get the hang of it.
And once you start to do math in your head, you start to see other applications where it might be useful, like in calculating how much tiling to get for your new kitchen floor when you go to hardware store. Or figuring out how far a tree will reach when it gets cut down; or how to convert baking measurements.
The more you do it the better at it you will get, so be patient and it will come to you.