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2013 AMC 10A Problem 4

Problem 4 of 25EasierNumber Theory

A softball team played ten games, scoring 1,1, 2,2, 3,3, 4,4, 5,5, 6,6, 7,7, 8,8, 9,9, 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?

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Solution

Note that if they scored twice as many runs as their opponents, then they scored an even number of runs. This means in the games where they scored 2,4,6,8,2, 4, 6, 8, and 1010 runs, their opponents scored 1,2,3,4,1, 2, 3, 4, and 55 runs respectively. This sums to 1+2+3+4+5=15. 1 + 2 + 3 + 4 + 5 = 15. In the other games, their opponents scored 2,4,6,8,2, 4, 6, 8, and 1010 runs. This sums to 2+4+6+8+10=30. 2 + 4 + 6 + 8 + 10 = 30. The total number of runs is then 15+30=45. 15 + 30 = 45. Thus, C is the correct answer.

More practice

Concepts: parity · casework

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.