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Number Theory Level 1

If the numbers from 1 onwards are written one after the other and a number of 20 digits is formed, what will be the digit in the unit's place of the number?


The answer is 1.

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Anuj Shikarkhane by Anuj Shikarkhane , 19, India

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4 solutions

Sunil Pradhan
Oct 23, 2014

one digit numbers used 1 to 9 = 1 × 9 = 9

2 digit numbers used = 20 – 9 = 11

so numbers used = 11/2 = 5 1/2

i.e. first digit of 6 th number

6 th 2 digit number = 10 + 5 = 15

so 20 th digit = 1

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Mehul Chaturvedi
Oct 19, 2014

12345678910111213141

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David Ng
Oct 18, 2014

1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 0, 1, 1, 1, 2, 1, 3, 1, 4, 1

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Fares Salem
Oct 18, 2014

the 20-digit's are all 1 so of course the unit is 1

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