2019 AMC 10B Problem 25

Below is the professionally curated solution for Problem 25 of the 2019 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AMC 10B solutions, or check the answer key.

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Concepts:arrangements with restrictionspartitions and compositionscombinations

Difficulty rating: 1770

25.

How many sequences of 00s and 11s of length 1919 are there that begin with a 0,0, end with a 0,0, contain no two consecutive 00s, and contain no three consecutive 11s?

55 55

60 60

65 65

70 70

75 75

Solution:

Our sequence starts with a 00 then has sequences of 110110 and 1010 in some order, where they each come after a 0.0.

Let the number of 110110 be xx and let the number of 1010 be y.y. Then the number of terms in the sequence is 3x+2y+1=19,3x+2y+1=19, making 3x+2y=18.3x+2y=18. The only possible ordered pairs are (x,y)=(6,0),(4,3),(2,6),(0,9).\begin{align*}(x,y) = &(6,0),\\&(4,3),\\&(2,6),\\&(0,9).\end{align*} Then, the number of ways to order the x+yx+y blocks is (x+yx),\binom{x+y}x, since we choose which xx of the x+yx+y positions hold a 110110 block.

Therefore, the total number of ways is (66)+(74)+(82)+(90)\binom 66 + \binom 74 + \binom 82 + \binom 90=1+35+28+1=1+35+28+1=65.=65.

Thus, the answer is C .

Problem 25 in Other Years