The following are some simple, but largely unknown math tricks you can do with numbers both humongous and small, which will indeed seem like magic to anyone watching you work! These math tricks will also will have the added benefit of helping you in a whole range of practical life situations, wherever numbers are needed! Hey who knows  one of these tricks may be the special ingredient that impresses the prospective employer of your dream job, or raises the eyebrow of that very special someone....
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Forget the traditional, cumbersome method your teacher taught you  and use this trick: to multiply a number by 5, divide by 2, and move the decimal point to the right. SO, if, say, you had the number 8,600 (the amount of money in your bank account right now??) and you wanted to multiply it by 5, simply do this:
Divide 8600 by 2 (easy!), which gives you 4300, and then just move the decimal point to the right, giving you 43000. You can do this very quickly in your head with just about any number, because dividing by two is usually pretty easy. And moving the decimal point is even easier! Just for fun, try multiplying 8600 by 5 in your head the traditional way. Well...it wasn't very fun, was it? Try multiplying a few more numbers by 5 in your head this way, until you can build up some magiclike speed. Here, use some of my numbers, or if you feel adventurous, come up with some of your own!
Don't let those odd numbers trip you up. You will end up with "something.5" when you divide by two, but after moving the decimal point over one space you get a number ending in 5. If it helps, just remember that all odd numbers multiplied by 5 end in 5!
Dividing by 5 is pretty much the opposite of multiplying by 5. In other words, you get two math magic tricks for pretty much the price of one! Here is the rule: to divide a number by 5, multiply by 2, and move the decimal point to the left. Sooo, this time if you want to divide the number 8,600 by 5, just do this:
Multiply 8600 by 2 (just double it  still easy!), which gives you 17200, and then just move the decimal point to the left, giving you 1720. Now try it the old way...I don't know about you, but doing that makes my brain hurt.... When you divide any number by 5 that doesn't end in a 5 or a zero, you will end up with a decimal point. Mental arithmetic involving decimal points usually brings on headaches of various potencies with people, but handling decimals this way is a piece of cake. It actually will enhance the magical aura surrounding you.... Watch this thought process at work:
123 ÷ 5 = [ Multiply 123 by 2 (double it), which is 246, and then move the decimal point to the left, which is 24.6 !] So the answer is 24.6. Doesn't 24.6 sound impressive??
Here's a quick overview of some other eerily simple math tricks involving multiplication that you can do for special situations:


Math Tricks & Math Magic