It is only a simple multiplication.

Number Theory Level 1

determine the answer without pen, paper or calculator's calculation,

111,111,111 X 111,111,111 = ?


The answer is 12345678987654321.

Devjeet Baghel by Devjeet Baghel , 22, India

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2 solutions

11×11=121 111×111=12321 1111×1111=1234321 The digit in the middle is determined by the no. Of 1s The digits increase by 1 till the middle digit and then decreases by 1

2 Helpful 0 Interesting 0 Brilliant 0 Confused

simple.. but tricky :) good one

Usman Rehman - 7 years, 3 months ago

good logic & excellent approach. vot up

azadali jivani - 5 years, 9 months ago

almost tricky... until I noticed...

I used to multiply 111 by 111 and resulted to 12321 (that middle number determines the number of 1's in both multiplicand and multiplier) if the multiplicand and the multiplier has the same number of 1's, then the answer will be likely going from 1 to any number not exceeding to 9. Since there are 9 1's displayed in both multiplicand and multiplier, the answer is from 1 going to 9 and return to 1 again (which is 12345678987654321).

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