2015 AMC 8 Problem 11

Attempt Problem 11 of the 2015 AMC 8 below, then check your answer against the video solution and professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2015 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

11.

In the small country of Mathland, all automobile license plates have four symbols. The first must be a vowel (A, E, I, O, or U), the second and third must be two different letters among the 2121 non-vowels, and the fourth must be a digit (0 through 99). If the symbols are chosen at random subject to these conditions, what is the probability that the plate will read “AMC8”?
122,050 \dfrac{1}{22{,}050}
121,000 \dfrac{1}{21{,}000}
110,500 \dfrac{1}{10{,}500}
12,100 \dfrac{1}{2{,}100}
11,050 \dfrac{1}{1{,}050}
Answer: B
Concepts:basic probabilitymultiplication principle
Small hint:
Count all license plates allowed by the rules.
Big hint:
Only one of those plates is exactly AMC8.
Video solution:
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Written solution:
There are 55 choices for the first symbol, 2121 choices for the second, 2020 choices for the third because it must be a different non-vowel, and 1010 choices for the final digit.
Thus there are 5⋅21⋅20⋅10=21,0005\cdot21\cdot20\cdot10=21{,}000 possible plates. Exactly one of these is AMC8, so the probability is 121,000.\dfrac{1}{21{,}000}.
Thus, B is the correct answer.
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