2013 AMC 12A Problem 2

Below is the professionally curated solution for Problem 2 of the 2013 AMC 12A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2013 AMC 12A solutions, or check the answer key.

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Concepts:paritycasework

Difficulty rating: 1010

2.

A softball team played ten games, scoring 1,2,3,4,5,6,7,8,9,1, 2, 3, 4, 5, 6, 7, 8, 9, and 1010 runs. They lost by one run in exactly five games. In each of their other games, they scored twice as many runs as their opponent. How many total runs did their opponents score?

3535

4040

4545

5050

5555

Solution:

The team can only score twice as many runs as its opponent when its own score is even. Those games have scores 2,4,6,8,10,2, 4, 6, 8, 10, so their opponents scored 1+2+3+4+5=15.1 + 2 + 3 + 4 + 5 = 15.

The other five games had scores 1,3,5,7,91, 3, 5, 7, 9 and were one-run losses, so their opponents scored 2+4+6+8+10=30.2 + 4 + 6 + 8 + 10 = 30. The total is 15+30=45.15 + 30 = 45.

Thus, the correct answer is C.

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