2011 AMC 10A Problem 14

Below is the professionally curated solution for Problem 14 of the 2011 AMC 10A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2011 AMC 10A solutions, or check the answer key.

All of the real AMC 8, AMC 10, AMC 12, and AIME problems in our complete solution collection are used with official legal permission of the Mathematical Association of America (MAA).

Concepts:dice (probability)circle areainequality

Difficulty rating: 1140

14.

A pair of standard 66-sided dice is rolled once. The sum of the numbers rolled determines the diameter of a circle. What is the probability that the numerical value of the area of the circle is less than the numerical value of the circle's circumference?

136\dfrac{1}{36}

112\dfrac{1}{12}

16\dfrac{1}{6}

14\dfrac{1}{4}

518\dfrac{5}{18}

Solution:

For the area to be less than the circumference, we must have πr2<2πr \pi r^2 \lt 2\pi r r<2. r \lt 2.

This means the diameter must be less than 4.4. There are three possible rolls that satisfy this: (1,1),(1,2),(2,1). (1, 1), (1, 2), (2, 1).

The probability is then 336=112.\dfrac{3}{36} = \dfrac{1}{12}.

Thus, B is the correct answer.

Problem 14 in Other Years