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Home  >>  CBSE XII  >>  Math  >>  Probability
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A lot of 100 watches is known to have 10 defective watches.If 8 watches are selected(one by one with replacement) at random,what is the probability that there will be at least one defective watch?

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1 Answer

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Toolbox:
  • \(In\;a\;batch\;of\;100\;watches\;10\;are\;defective\;\)
  • \(p(a\;watch\;selected\;is\;defective)=\large\frac{10}{100}=\frac{1}{10}\)
  • \(p=\large\frac{1}{10}\;q=1-\frac{1}{10}=\frac{9}{10}\)
  • \(8\;watches\;are\;selected\;n=8\)
  • \(p=(X=r)\;=\;\Large\;c^n_r\;p^r\;q^{n-r}\)
Answer:
\(p(at\;least\;1\;defective\;)=p(X\;\geq\;1)\)
=\(1-p(X\;<\;1)\)
=\(1-\;p(X\;=\;0)\)
\(=\Large\;1-c^8_o\;(\frac{1}{10})^0\;(\frac{9}{10})^8\)
\(=\Large\;1-(\frac{9}{10})^8\)

 

answered Mar 18, 2013 by balaji.thirumalai
 

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