1998 AIME Problem 2

Attempt Problem 2 of the 1998 AIME 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 1998 AIME solutions, or check the answer key.

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2.

Find the number of ordered pairs (x,y)(x, y) of positive integers that satisfy x2y60x \le 2y \le 60 and y2x60.y \le 2x \le 60.

Answer: 480
Concepts:counting pairsinequalitycomplementary countingsymmetry
Difficulty rating: 2110
Solution:

The chains unpack into four conditions: x2y,x \le 2y, 2y60,2y \le 60, y2x,y \le 2x, and 2x60.2x \le 60. So (x,y)(x, y) lies in the square 1x,y30,1 \le x, y \le 30, and within it we must avoid x>2yx \gt 2y and y>2x,y \gt 2x, which cannot both happen.

Pairs with y>2x:y \gt 2x: for each xx from 11 to 1414 the values y=2x+1,,30y = 2x + 1, \ldots, 30 work, giving x=114(302x)\sum_{x=1}^{14} (30 - 2x) =420210= 420 - 210 =210= 210 pairs. By the symmetry swapping xx and y,y, there are also 210210 pairs with x>2y.x \gt 2y.

The answer is 3030210210=480.30 \cdot 30 - 210 - 210 = 480.

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