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Number theory Difficulty 5.0 AIME Prove it Estonia

Determine all pairs (m,n)(m, n) of natural numbers that satisfy mn=96m - n = 96 and lcm(m,n)=2024\text{lcm}(m, n) = 2024.

Solutions — 2

Solution 1

As lcm(m,n)=2024=8253\text{lcm}(m,n) = 2024 = 8 \cdot 253 and 8=238 = 2^3, at least one of the numbers mm and nn is divisible by 88. Since 896=mn8 \mid 96 = m-n, the other one must also be divisible by 88. Both mm and nn are divisors of 20242024. All divisors of 20242024 that are divisible by 88 are 88, 8888, 184184 and 20242024. The only two of these with difference 9696 are 184184 and 8888. A straightforward check shows that lcm(184,88)=lcm(823,811)=82311=2024\text{lcm}(184,88) = \text{lcm}(8 \cdot 23, 8 \cdot 11) = 8 \cdot 23 \cdot 11 = 2024 indeed.

Solution 2

As lcm(m,n)=2024\text{lcm}(m,n) = 2024, both mm and nn are divisors of 20242024. All divisors of 20242024 are
1, 2, 4, 8, 11, 22, 23, 44, 46, 88, 92, 184, 253, 506, 1012, 2024.
As mn=96m-n=96, we must have m>96m > 96. This observation cuts out all cases except m=184,m=253,m=506,m=1012m = 184, m = 253, m = 506, m = 1012 and m=2024m = 2024. As differences between 253253, 506506, 10121012 and 20242024 are larger than 9696, it suffices to check the options m=184m = 184 and m=253m = 253 which imply n=88n = 88 and n=157n = 157, respectively. As 1572024157 \nmid 2024, the only solution can be (m,n)=(184,88)(m,n) = (184, 88). An easy check shows that lcm(184,88)=lcm(823,811)=82311=2024\text{lcm}(184,88) = \text{lcm}(8 \cdot 23, 8 \cdot 11) = 8 \cdot 23 \cdot 11 = 2024 indeed.

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