How many binary sequences of length 7 have at least two 1's? Can someone please explain the procedure in detail please. I tried solving it using the "count what you do not want" procedure, but I got nowhere. Thank you in advance
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- 4$\begingroup$ What do you get if you count those that have exactly zero or exactly one 1? $\endgroup$fuglede– fuglede2015-02-09 17:16:27 +00:00Commented Feb 9, 2015 at 17:16
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1 Answer
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1 The number of $7$-digit sequences is $2^7=128$
The number of $7$-digit sequences with $0$ occurrences of "one" is $\binom70=1$
The number of $7$-digit sequences with $1$ occurrence of "one" is $\binom71=7$
The number of $7$-digit sequences with $2$ or more occurrences of "one" is $128-(1+7)=120$
- $\begingroup$ @MathLover95: You're welcome :) $\endgroup$barak manos– barak manos2015-02-09 18:43:08 +00:00Commented Feb 9, 2015 at 18:43