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10 10 coins are flipped randomly by Bob, and then placed in a line.

For an arrangement of coins S S , suppose s s is a subset of consecutive elements of S S (meaning that they appear next to each other in the line). Let U ( s ) U(s) and D ( s ) D(s) be the number of coins facing up and down in s s , respectively. S S is considered to be n c l o s e n-close if for any s s , U ( s ) D ( s ) n \left| U(s)-D(s) \right| \le n .

Given that an arrangement S S is 3 c l o s e 3-close , what is the probability that it is also 2 c l o s e 2-close ? Express your answer as m n \frac { m }{ n } where m m and n n are relatively prime natural numbers. What is m + n m+n ?

Tip: There's a nice, simple way to solve this without casework. Don't make things too complicated!


The answer is 233.

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