A number theory problem by A Former Brilliant Member

Number Theory Level 1

If you write down all the numbers from 1 to 100, then how many times do you write 4 ?


The answer is 20.

A Former Brilliant Member by A Former Brilliant Member

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3 solutions

4 , 14 , 24 , 34 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 54 , 64 , 74 , 84 , 94 4,14,24,34,40,41,42,43,44,45,46,47,48,49,54,64,74,84,94

There are 20 20 fours.

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Sophie Crane
Oct 31, 2014

It appears 10 times as a units digit and 10 times as a tens digit.

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Raiyun Razeen
Oct 31, 2014

It is known that all the numbers from 1 to 9 come 20 times if all the numbers from 1 to 100 is written down.

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you can count how many times it comes using the principle of permutation or counting.

Harshita Moondra - 6 years, 6 months ago

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