Question

Let $S$ be the set of all integers $z \gt 1$ such that for all pairs of nonnegative integers $(x, y)$ with $x \lt y \lt z,$ the remainder when $2025x$ is divided by $z$ is less than the remainder when $2025y$ is divided by $z.$ What is the sum of the elements of $S?$

$3041$

$3542$

$3750$

$4044$

$4319$

Answer: E

Concepts:modular arithmetic divisibility sum of factors

Difficulty rating: 2380

Solution:

The condition requires $k \mapsto 2025k \bmod z$ to be strictly increasing on $\{0, 1, \ldots, z-1\}.$ A strictly increasing list of $z$ distinct values in $[0, z-1]$ must be $0, 1, \ldots, z-1,$ so $2025 \equiv 1 \pmod z,$ i.e. $z \mid 2024.$ Since $2024 = 2^3 \cdot 11 \cdot 23,$ the sum of all its divisors is $(1+2+4+8)(1+11)(1+23)$ $= 15 \cdot 12 \cdot 24$ $= 4320.$ Excluding $z = 1$ leaves $4319.$

Thus, the correct answer is E.

2025 AMC 12B Problem 23

Answer: E

Difficulty rating: 2380

Solution:

Problem 23 in Other Years