The Math Magic Page

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....


Contents:



Multiplying any number by 5

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 magic-like speed. Here, use some of my numbers, or if you feel adventurous, come up with some of your own!

750 x 5 = ? 66 x 5 = ? 888 x 5 = ?
42 x 5 = ? 4242 x 5 = ? 125 x 5 = ?
37 x 5 = ? 96200 x 5 = ? 330 x 5 = ?

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 any number by 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??


Ok. Now to get the speed up. Try some of these:

750 ÷ 5 = ? 66 ÷ 5 = ? 444 ÷ 5 = ?
42 ÷ 5 = ? 4242 ÷ 5 = ? 125 ÷ 5 = ?
370 ÷ 5 = ? 92000 ÷ 5 = ? 330 ÷ 5 = ?





Other multiplication math tricks

Here's a quick overview of some other eerily simple math tricks involving multiplication that you can do for special situations:


Multiplying a number by .5, .05, 50, 500 etc.Multiplying with these numbers is precisely the same as multiplying a number by 5 (see above), except that you move the decimal point differently. For example, when multiplying by 50, divide the other number by two, then move the decimal point two spaces to the right, instead of one.
Multiplying a number by 2Convert it to the corresponding addition question, and then if necessary break it down into steps, adding the columns separately. So, if you have 57 x 2, convert it to 57 + 57 (which by itself is usually easier than thinking "x 2"), and then add the tens columns together (50 + 50 = 100) and then add that to the sum of the ones columns (7 + 7 = 14), and you get the simple equation 100 + 14 = 114.
Usually though, you can just think "double the number", and it will be easy enough to do in your head just like that.
Multiplying a number by 4Just double the number twice. Makes sense, no? (2 x 2 = 4   -- duh!)
For example, 53 x 4 would be: 53 doubled = 106, then 106 doubled = 212. Therefore 53 x 4 = 212.
Multiplying a number by 5As stated above, divide the number by 2 and move the decimal point one space to the right (making the number larger). Example: 47 x 8 would be: 47 doubled = 94; 94 doubled = 188; 188 doubled = 376. Therefore, 47 x 8 = 276.
Multiplying a number by 8Double the number progressively three times (2 x 2 x 2 = 8). This will get considerably more difficult with larger numbers, of course!
Example: 47 x 8 would be: 47 doubled = 94; 94 doubled = 188; 188 doubled = 376. Therefore, 47 x 8 = 276.
Multiplying a number by 9A couple of possibilities here...you could multiply the number by 3, and then by 3 again, or even sneakier, you can multiply the number by 10, and then simply subtract the original number from it.
For example, if you had 9 x 42, in your head do 10 x 42 (= 420) and subtract 42 from it (420 - 42 = 378).
Multiplying a number by 10Add a "zero" at the end. Not too hard....
Multiplying a number by 11Like the trick for 9, multiply the number by 10, then add the original number to it.
For example, for 57 x 11:   57 x 10 = 570, then 570 + 57 = 627. Therefore, 57 x 11 = 627.
Multiplying a number by 12Just like 11, only add double the original number to it.
So, for 57 x 12:   57 x 10 = 570; then 57 doubled = 114; and finally 570 + 114 = 684.
Special note: Since multiplying a number by 10 doesn't take much in the way of "thought", it is probably less taxing on your memory to do this in the reverse order, i.e. 57 doubled + 570 (still = 684 !)



Want more? Here are a few very good, inexpensive books that are packed with math magic-like tricks like these! You should definitely pick them up:

(In association with Amazon.com.)

 


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Math Tricks & Math Magic