2025 AIME I Problem 10

Attempt Problem 10 of the 2025 AIME I 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 AIME I solutions, or check the answer key.

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10.

The 2727 cells of a 3×93 \times 9 grid are filled in using the numbers 11 through 99 so that each row contains 99 different numbers, and each of the three 3×33 \times 3 blocks heavily outlined in the example below contains 99 different numbers, as in the first three rows of a Sudoku puzzle.

The number of different ways to fill such a grid can be written as paqbrcsd,p^a \cdot q^b \cdot r^c \cdot s^d, where p,p, q,q, r,r, and ss are distinct prime numbers and a,a, b,b, c,c, dd are positive integers. Find pa+qb+rc+sd.p \cdot a + q \cdot b + r \cdot c + s \cdot d.

Answer: 81
Concepts:arrangements with restrictionsmultiplication principleprime factorization
Difficulty rating: 2990
Solution:

Fill the left block arbitrarily: 9!9! ways. Let R1,R2,R3R_1, R_2, R_3 be the sets of three digits in its rows. In the middle block, row ii must avoid RiR_i (those digits already appear in row ii), and the block's three rows must partition {1,,9}.\{1, \ldots, 9\}. Say its top row takes jj digits from R2R_2 and 3j3 - j from R3.R_3. Balancing the three rows then forces the middle row to take 3j3 - j digits from R1R_1 together with all jj remaining digits of R3,R_3, and the bottom row is determined. The number of content choices is j=03(3j)(33j)2=1+27+27+1=56. \begin{gathered} \sum_{j=0}^{3} \binom{3}{j}\binom{3}{3-j}^2 \\ = 1 + 27 + 27 + 1 \\ = 56. \end{gathered}

The right block's row contents are then forced (row ii takes whatever is missing from row ii), and each of the six rows of the middle and right blocks can be ordered internally in 3!3! ways. The total is 9!5666=(273457)(237)(2636)=2163105172. \begin{gathered} 9! \cdot 56 \cdot 6^6 \\ = (2^7 \cdot 3^4 \cdot 5 \cdot 7)(2^3 \cdot 7)(2^6 \cdot 3^6) \\ = 2^{16} \cdot 3^{10} \cdot 5^1 \cdot 7^2. \end{gathered}

Therefore pa+qb+rc+sdp \cdot a + q \cdot b + r \cdot c + s \cdot d =216+310+51= 2 \cdot 16 + 3 \cdot 10 + 5 \cdot 1 +72=81.+ 7 \cdot 2 = 81.

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