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2019 AMC 10A Problem 4

Problem 4 of 25EasierAlgebraCounting & Probability

A box contains 2828 red balls, 2020 green balls, 1919 yellow balls, 1313 blue balls, 1111 white balls, and 99 black balls. What is the minimum number of balls that must be drawn from the box without replacement to guarantee that at least 1515 balls of a single color will be drawn?

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Solution

Note that we can pull as many as 1414 balls of each color without ensuring that 1515 balls of one color are drawn. This means that we can draw all of the black, white and blue balls, along with 1414 red, green, and yellow balls. This gives us a total of 9+11+13+314 9 + 11 + 13 + 3 \cdot 14 =33+42= 33 + 42 =75.= 75. We need to add one at the end, however, to ensure that we get that 1515th ball of some color, 75+1=76.75 + 1 = 76. Thus, B is the correct answer.

More practice

Concepts: pigeonhole principle · extremal argument

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