Okay, so let's just get this one out of the way first - there are calculators, and they're actually pretty amazing at doing maths. But it's time to dust that tired, old brain off so we can learn a great little arithmetic trick, courtesy of the latest episode of MinutePhysics.
When adding two large numbers together, instead of bothering with all that long division and borrowing stuff, says Henry, you can just replace each digit of the smaller number with nine minus that digit, except the final digit, which you'll subtract from 10.
If you add your new number to the number you were going to subtract the original smaller number from, you'll actually get the right answer. If you ignore the first digit, of course.
It sounds weirdly complicated, but watch the video above to see how simple it actually is. And it just so happens to work for any positive numbers, so be sure to commit this little trick to memory, for all those times you don't have a calculator on hand.