2015 AMC 10A Problem 2

Attempt Problem 2 of the 2015 AMC 10A 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 2015 AMC 10A solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

2.

A box contains a collection of triangular and square tiles. There are 2525 tiles in the box, containing 8484 edges total. How many square tiles are there in the box?
33
55
77
99
1111
Answer: D
Concepts:system of equations
Small hint:
Compare the total to the case where all tiles are triangular
Big hint:
Each square contributes one more edge than a triangle
Solution:
Let xx be the number of triangular tiles and yy be the number of square tiles. We then have that x+y=25 x + y = 25 and 3x+4y=84. 3x + 4y = 84. Multiplying the first equation by 33 gives us 3x+3y=75 3x + 3y = 75 and subtracting this from the other equation gives us y=9.y = 9.
Thus, D is the correct answer.
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