Multiplying a number by 5

Tip: Divide the number by 2 and add a "Zero".

suppose the no is 300 . divide the no by 2 = 150 and add a "zero". So the ans= 1500

another example: 244 * 5 = 1220 .

Mutiplying a number by 11

Tip : For 2 digit nos Simply add the first and second digits and place the result between them.

Eg. 24 * 11 = 264

another eg : 79*11 = units place =9 , here 7+9= 16 so write 6 in middle..
carry fwd 1 and add to 7 so the ans will be 869

Sq root trick with nos above 100

Needs memorising of last digits of squares from 1-9

Here is the list

1---------->1
2---------->4
3---------->9
4---------->6
5---------->5
6---------->6
7---------->9
8---------->4
9---------->1

suppose the no is 169, then take the hundreds value (over here 100 and key in its sq rt ie 10)

now observe the last digit . Thats the same ending for 3 and 7 . so the no is either 10+3=13
Or 10+7= 17 . To determine the correct answer , take average of the sq of extreme digits ie
over here (10'2 + 20'2) /2 = 250. In our case 169<250 . Therefore the answer will be 13 .

If here the problem number was greater than 250 then the answer would have been 17

The Cube Root Trick

Tricks, Rules & MethodsIf someone cubed a two-digit number on a calculator and gave you the result - but not the original number - could you extract the cube root? With this trick, you'll be able to do just that - instantly!
A bit of homework is required for this trick, but it's worth the effort if you like to show off.

First, memorize the cubes of the digits 1 through 9:

1, 8, 27, 64, 125, 216, 343, 512, 729.

Next memorize the "endings" of the cubes. For example, the ending of 93 is 9, because 93 = 729. The "ending" (or last digit) is 9.

So let's make a list. "1 cubed ends in 1" is abbreviated "1 --> 1".

1 --> 1
2 --> 8
3 --> 7
4 --> 4
5 --> 5
6 --> 6
7 --> 3
8 --> 2
9 --> 9

These are easily memorized. 1 and 9 (at the extremes) are "self-enders", as are the 4, 5, and 6 (in the center). The others involve "a sum of 10": 2 ends in 8, 8 ends in 2, 3 ends in 7, and 7 ends in 3.

Now how to do the trick!
Tell a friend to secretly pick any two-digit number and then have him or her use a calculator to cube it. Let's say he picks 76. So using the calculator he computes 76 x 76 x 76 . He then tells you the cube: 438,976.

To instantly determine his original number (ie, compute the cube root), follow these easy steps:

1. Drop the last three digits and find the largest cube contained in 438. This is 73 = 343, so the tens-digit is 7.
(This is why you had to memorize the cubes of the digits 1 through 9)
2. Now go back to the last three digits. Look at the last digit, 6. That's the same ending as 63, so your units-digit is 6.
(This is why you had to memorize the "endings" of the cubes for digits 1 through 9)

So the cube root of 438,976 is 76

Another example:
Let's say your friend chooses a secret two-digit number whose cube is 79,507. How do you instantly determine the cube root?

1. Drop the last three digits and find the largest cube in 79. This is 4^3 = 64, so the tens-digit of the cube root is 4.
2. Now go back to the last three digits. Look at the last digit, 7. That's the same ending as 3-cubed. So the units-digit of your cube root is 3.

Therefore, the cube root of 79,507 is 43.