[MENTAL ABILITY] DO SPEEDY MATH IN YOUR HEAD

9 times any digit number (the super simple way)

Take the number you wish to multiply by 9, multiply it by 10 and substract the original number.

Let us take 23 for instance:

23 * 9 = (23 * 10) - (23 * 1) = 230 - 23 = 207

11 times any digit number (the super simple way)

This is just the same principle as 9 times any digit number.

Take the number you wish to multiply by 11, multiply it by 10 and add the original number once more.

Let us take 23 for instance:

23 * 11 = (23 * 10) + (23 * 1) = 230 + 23 = 253

11 times any digit number (the simpler way)

Take any number you want to multiply by 11, like 18 for example

Write 18 then shift 18 one left space over:

 18

18

And add the numbers up in the middle column to get 198 as the sum. 1 (1+8 = 9) 8

If it is a double digit number like 88, where it's:

 88

88

Then add the middle 8s to be 16, and carry the 1 over to the left for:

968

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11 times any two-digiter

To multiply, say, 11 x 32, add the digits of 32 (3 + 2 = 5) and insert the sum between them: 352. Numbers with two-digit sums use a slight variation: For 11 x 84 (8 + 4 = 12), add the 1 from 12 to the 8 and leave the 2 in the middle: 924.

Square off

To square a number like 14, identify the preceding round number — in this case, 10. Since you subtracted 4 to get 10, add 4 to 14 to get 18 and multiply that by 10 (*the round number). Add to that the square of 4: 180 + 16 = 196.

Proof

(x - y)(x + y) + y^2 = x^2

x^2 + xy - xy - y^2 + y^2 = x^2

x^2 = x^2

To find the square of the next number in a series without multiplying: Say you know the square of 12 is 144. Add 12 to 144 and add the next number — in this case, 13 — to find the square of 13: 144 + 12 = 156 + 13 = 169, the square of 13; and then, 169 + 13 = 182 + 14 = 196, etc.

Magic number

Ask a fan to think of any number. Then have them double it, add 12, divide by 2, and subtract the original number. Before they're done, tell them the answer: 6. It will always be 6.

Contributed by Katharine Gammon

The answer will always be one-half of any number the fan adds after doubling (... add 120 ... the answer will be 60; ... add 60 ... the answer will be 30). You can have the fan also think of the number to add, but the fan must tell you that number.

((2x + 2y)/ 2) - x = y. So magical. BTW I don't think the person would be your fan much longer if they were asked to do this in their head.

Fives Times

If you have a multiplication of five, then an easy way to find the answer is to half the multiplied number and times by ten.

For example: 24 * 5 = (24 / 2) * 10 = 120

Contributed by Luke Jones'

Squaring Numbers that End in Five

To square a number that ends in 5 (e.g., 25, 65, 1005) simply remove the 5 from the end of the number and multiply the remaining number times the next number then tack a '25' on the end of it (squares of numbers that end in 5 will always end in 25).

For example,

25 squared is 2*3 = 6 and tack on '25' to the end = 625.
65 squared is 6*7 = 42 and tack on '25; to the end = 4225.

Contributed by Chris Merritt

Multiplying by 8

To multiply by 8 just multiply by 2 3 times.

e.g. 37 * 8 = (37 * 2) *2) *2) = 296

Actual Calculation done in head.

(2 * 7) + (2 * 30) = 70 + 4 = 74
(2 * 4) + (2 * 70) = 140 + 8 = 148
(2 * 8) + (2 * 140) = 280 + 16 = 296

COURTESY:MRINAL PATEL