2025 AMC 12B Problem 2

Attempt Problem 2 of the 2025 AMC 12B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2025 AMC 12B solutions, or check the answer key.

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2.

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

90259025

90709070

90909090

91159115

91609160

Answer: D
Concepts:units digitpattern recognitionsummation
Difficulty rating: 1020
Small Hint:

The ones digit of n2n^2 depends only on the ones digit of n,n, so the list repeats every 1010 terms

Big Hint:

Sum one full block of 10,10, multiply by the number of blocks, then add the leftover terms

Solution:

The ones digits of 12,22,,1021^2, 2^2, \ldots, 10^2 are 1,4,9,6,5,6,9,4,1,0,1, 4, 9, 6, 5, 6, 9, 4, 1, 0, which sum to 45.45. The 20252025 terms contain 202202 full blocks (20202020 terms) plus 55 more with digits 1,4,9,6,51, 4, 9, 6, 5 summing to 25.25. The total is 20245+25=9090+25202 \cdot 45 + 25 = 9090 + 25 =9115.= 9115.

Thus, the correct answer is D.

Problem 1#1
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