**Some
interesting math puzzles**

**From the
collection of P. Narayanan (Amazing Vijay’s father)**

**The 9 principle:**

- Puzzle: Write down any 3 digit number. Reverse it. Subtract the smaller number from the larger number. Now if you tell me the first or the last digit, I can tell you the other two digits of the answer.

Answer: The answer will always be x9y and first and last digit will add up to 9 also. So tell them that the middle digit is 9 and then subtract from 9 to come up with the other digit

Puzzle: Write down any 3 digit number. Reverse it. Subtract the smaller number from the larger number. Now reverse the answer and add the 2 numbers together. I can tell you the answer.

Answer: It is always 1089

**Quick calculations:**

Finding the square of any number ending with 5.

First digit x (first digit+1) and then last 2 digits are always 25

E.g 35 x 35 = 3x4 = 12, so the answer is 1225

E,g 85 x 85 = 8x9 = 72, so answer is 7225

Finding the square of any numbers starting with 5

First 2 digits are (25+second digit) ; Last 2 digits are the square of the second digit

E.g. 56 x 56 = 25+6 = 31, so the answer is 3136

E.g. 52 x 52 = 25+2 = 27, so the answer is 2704

Multiplying by 11.

Write down the last digit, Add 2 digits and write down answer to the left. Write first digit to left of this.

E.g. 23 x 11 = 2 (2+3)=5) 3 ->253

Sometimes you have to carry over – but then this may not be so quick

E.g. 98 x 11 = (9+1 (carry)) (9+8)=7 and carry 1 8 -> 1078

What is GTTTT ?

If you know Tamil, the answer is “Oru G nalu T” or “Originality”

Did you know that the letters A, B, C or D do not appear in the spelling of any number from 1 to 99.

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