6 Easy Arithmetic Tricks

Math can be terrifying for many people. The tricks shown below would hopefully improve your general knowledge of mathematics and your speed when you do math in your head.

1. Multiply by 11
We all know the trick when multiplying by ten - add 0 to the end of the number, but did you know there is an equally easy trick for multiplying a two digit number by 11? Here’s how:

Take the original number and imagine a space between the two digits. In this example we will use 62.

6_2

Add the two numbers together and put them in the middle: 6_(6+2)_2

Answer: 682

If the numbers in the middle add up to a 2 digit number, just insert the second number and add 1 to the first. In this example we will use 89.

Step 1: 8_(8+9)_9
Step 2: 8_(17)_9
Step 3: (8+1)_7_9
Step 4: 9_7_9

Answer: 979

* Works with two digit numbers only.


2. Multiply by 9
This one is simple.

To multiple any numbers between 1 and 9 by 9 hold both hands in front of your face

Drop the finger that corresponds to the number you are multiplying (for example 9 × 4, drop your fourth finger)

Count the fingers before the dropped finger (in the case of 9 × 4 it is 3) then count the numbers after (in this case 6)

Answer: 36

* Works well when no calculator is at hand (pun intended).


3. Multiply by 5

Most people memorize the 5 times tables very easily, but when you get in to larger numbers it gets more complex - or does it? This trick is super easy.

Take any number, then divide it by 2 (in other words, halve the number). If the result is whole, add a 0 at the end. If it is not, ignore the remainder and add a 5 at the end. It works every time:

Step 1: 1782 x 5 = (1782 / 2) & 5 or 0

Step 2: 1782 / 2 = 891 (whole number so add 0)

Answer: 8910

Let's try another:

Step 1: 3997 x 5 = (3997 / 2)

Step 2: 1998.5 (fractional number (ignore remainder, add 5)

Answer: 19985


4. Dividing by 5

Dividing a large number by five is actually very simple. All you do is multiply by 2 and move the decimal point:

255 / 5

Step1: 255 x 2 = 510

Step2: Move the decimal: 51.0 or just 51

Answer: 51

Let's try another:

2009 / 5

Step 1: 2009 x 2 = 4018

Step2: 401.8

Answer: 401.8


5. Quick Square

If you need to square a 2 digit number ending in 5, you can do so very easily with this trick. Multiply the first digit by itself + 1, and put 25 on the end. That is all!

652 = (6x(6+1)) & 25

= (6 x 7) & 25

= 42 & 25

Answer: 4225

* Works with two digit numbers only.



6. Subtracting from 000s

To subtract a large number from 100, 1000 or 1000000 you can use this basic rule: subtract all but the last number from 9, then subtract the last number from 10:

1000

-526

Step1: subtract 5 from 9 = 4

Step2: subtract 2 from 9 = 7

Step3: subtract 6 from 10 = 4

Answer: 474