2009 AMC 10B Problem 1

Below is the professionally curated solution for Problem 1 of the 2009 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2009 AMC 10B solutions, or check the answer key.

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Concepts:modular arithmetic money

Difficulty rating: 720

Each morning of her five-day workweek, Jane bought either a $50$ -cent muffin or a $75$ -cent bagel. Her total cost for the week was a whole number of dollars. How many bagels did she buy?

$1$

$2$

$3$

$4$

$5$

Solution:

If Jane buys $b$ bagels, she buys $5-b$ muffins, for a total of $50(5-b)+75b = 250+25b$ cents. This is a whole number of dollars when $250+25b$ is a multiple of $100,$ that is, when $25b\equiv50\pmod{100},$ or $b\equiv2\pmod4.$

The only value with $0\le b\le5$ is $b=2.$

Thus, the correct answer is B.

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