Maths Olympiad Prep

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Number theory Difficulty 6.0 National Olympiad Prove it Italy

Problem:

Determine for which positive integers nn there exists a positive integer AA such that
- AA is a multiple of 2022,
- the decimal expression of AA contains only digits 0 and 7,
- the decimal expression of AA contains the digit 7 exactly nn times.

Solution

Solution:

The required values of nn are all and only the multiples of 3.

Necessary condition Let AA be a multiple of 2022 whose decimal expression contains the digit 7 exactly nn times, and possibly other digits 0. Since 2022 is a multiple of 3, AA must also be a multiple of 3, and therefore (by the divisibility criterion for 3) the sum of the digits of AA must in turn be a multiple of 3. Now the sum of the digits of AA is 7n7n, and this is a multiple of 3 only if nn is a multiple of 3.

Sufficient condition We observe that the number 70770=20223570770 = 2022 \cdot 35 is a multiple of 2022 and its decimal expression contains three digits 7 and two digits 0. This shows in particular that n=3n=3 has the required property.

More generally, for every positive integer kk we observe that the number written by repeating kk times the block of five digits 70770, that is, the number
707707077070770k volte=70770(1+105+1010++105(k1)) \underbrace{7077070770 \ldots 70770}_{k \text{ volte}} = 70770 \cdot \left(1 + 10^{5} + 10^{10} + \ldots + 10^{5(k-1)}\right)
is in turn a multiple of 2022 and its decimal expression contains the digit 7 3k3k times and the digit 0 2k2k times. This shows that every multiple of 3 has the required property.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.