2018 AMC 10B Problem 6

Below is the professionally curated solution for Problem 6 of the 2018 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2018 AMC 10B solutions, or check the answer key.

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Concepts:sampling without replacementbasic probability

Difficulty rating: 1290

6.

A box contains 55 chips, numbered 1,2,3,4,1, 2, 3, 4, and 5.5. Chips are drawn randomly one at a time without replacement until the sum of the values drawn exceeds 4.4. What is the probability that 33 draws are required?

115\dfrac{1}{15}

110\dfrac{1}{10}

16\dfrac{1}{6}

15\dfrac{1}{5}

14\dfrac{1}{4}

Solution:

We need a third draw exactly when the first two chips still sum to 44 or less. The only such pairs are {1,2}\{1,2\} and {1,3}.\{1,3\}. Each shows up as an ordered pair of first draws in 22 ways, so there are 44 favorable sequences out of 54=205 \cdot 4 = 20 equally likely ones. The probability is 4/20=15.4/20 = \tfrac15. Therefore, the answer is D.

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