2010 AMC 10B Problem 5

Attempt Problem 5 of the 2010 AMC 10B 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 2010 AMC 10B solutions, or check the answer key.

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5.

A month with 3131 days has the same number of Mondays and Wednesdays. How many of the seven days of the week could be the first day of this month?

22

33

44

55

66

Answer: B
Concepts:date and timemodular arithmetic
Difficulty rating: 960
Solution:

Note that 3131 days leaves a remainder of 33 when divided by 7.7.

This means that if the month starts on a Saturday, Sunday, Tuesday, or Wednesday, there will be an uneven number of Mondays and Wednesdays.

Then the month can only start on a Monday, Thursday, or Friday.

Thus, B is the correct answer.

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