2015 AMC 10A Problem 20

Below is the professionally curated solution for Problem 20 of the 2015 AMC 10A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2015 AMC 10A solutions, or check the answer key.

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Concepts:areaperimeterSimon’s Favorite Factoring Trick

Difficulty rating: 1540

20.

A rectangle with positive integer side lengths in cm\text{cm} has area AA cm2\text{cm}^2 and perimeter PP cm.\text{cm}. Which of the following numbers cannot equal A+P?A+P?

100100

102102

104104

106106

108108

Solution:

Let the side lengths be positive integers xx and yy. Then A+P=xy+2x+2y=(x+2)(y+2)4.A+P=xy+2x+2y=(x+2)(y+2)-4. Hence A+P+4A+P+4 must factor into two integers both at least 33.

The answer choices plus 44 are 104,106,108,110,112104,106,108,110,112. All except 106106 have a factorization with both factors at least 33: 104=426,108=912,110=1011,112=716.104=4\cdot26, \quad 108=9\cdot12, \quad 110=10\cdot11, \quad 112=7\cdot16. But 106=253106=2\cdot53, so it cannot equal (x+2)(y+2)(x+2)(y+2).

Thus, B is the correct answer.

Problem 20 in Other Years