1998 AIME Problem 2

Below is the professionally curated solution for Problem 2 of the 1998 AIME, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1998 AIME solutions, or check the answer key.

All of the real AMC 8, AMC 10, AMC 12, and AIME problems in our complete solution collection are used with official legal permission of the Mathematical Association of America (MAA).

Concepts:counting pairsinequalitycomplementary countingsymmetry

Difficulty rating: 2110

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.

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)=420210=210\sum_{x=1}^{14} (30 - 2x) = 420 - 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.

← Problem 1Full ExamProblem 3

Problem 2 in Other Years