To be fair, there are definitely some teachers who told students about this trick. But if yours didn't, here's how you can easily remember your times table for multiplying by nine. It's so simple, it almost feels like cheating.
Using this "Z" trick can help you calculate a fraction of a whole number. Not only is it helpful, but it also makes you feel like Zorro himself. But only if Zorro focused less on sword fighting and more on acing his math test.
SPOILER ALERT: If your age is 30, the answer is 303030. If your age is 58, the answer is 585858, and so on. It works because 259 times 39 equals 10101, and multiplying a two-digit number by that number results in it repeating three times.
If you're multiplying two numbers up in the 90s, this trick can help. If you add up how far the two numbers are below 100, you can subtract that new number from 100 to get your first two digits. Then multiply how far the numbers are below 100, and you get the final two digits.
If you need to multiply a two-digit number by 11, you can use this trick if the sum of its digits is under 10. Just add the two digits together, then place that inside the two digits, and the answer magically appears!
You probably know you can add on your fingers, but you can also multiply on them; in particular, when you are multiplying numbers 6-10 by each other. For this, think of the fingers on each hand as being 10 through 6. Then have the fingers of the two numbers you're multiplying touch each other (in this example, it's 7x8). The number of fingers above the intersection is 5. The number of fingers at or below the intersection is 2 times 3, or 6. Put those numbers together and you get 56. (Sound complicated? Go here to see it in more detail.)
This is an alternate way to multiply two numbers together. In this example, 12 is represented by the black lines (one line, then two more), and 13 is represented by the blue lines (one line on the bottom, then three more on top). Once you have your lines, count the number of spots where the lines intersect, and you get your answer.
It can be easy to mix up the "greater than" symbol with the "less than" symbol. If you have trouble keeping them straight, you can think of the symbols as alligator mouths. The alligator will want to eat the number that's bigger, which is why the top alligator is the "less than" symbol, and the bottom one is "greater than."
If you need to multiply something by 15, here's a simple way to do it in your head. And if you want to multiply by 20, just add a zero to the number, then double it. If you weren't aware of this trick before, knowing how much to tip at a restaurant just got a lot easier.
And here's another one that might not actually be beneficial in the real world. But if you ever want to make a bar bet that you can calculate 111,111,111 squared without using your calculator, you can totally win.