2025 AMC 10B Problem 2

Below is the professionally curated solution for Problem 2 of the 2025 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2025 AMC 10B solutions, or check the answer key.

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Concepts:units digitperfect squarepattern recognition

Difficulty rating: 990

2.

Jerry wrote down the ones digit of each of the first 20252025 positive squares: 1,4,9,6,5,6,1, 4, 9, 6, 5, 6, \ldots What is the sum of all the numbers Jerry wrote down?

90259025

90709070

90909090

91159115

91609160

Solution:

The ones digit of n2n^2 depends only on the ones digit of n,n, so the list repeats every 1010 terms: 1,4,9,6,5,6,9,4,1,0.1, 4, 9, 6, 5, 6, 9, 4, 1, 0. One block sums to 45.45. Now 2025=20210+5,2025 = 202 \cdot 10 + 5, so we get 202202 full blocks plus the first five terms: 20245+(1+4+9+6+5)=9090+25=9115.202 \cdot 45 + (1 + 4 + 9 + 6 + 5) = 9090 + 25 = 9115. Therefore, the answer is D.

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