Question
Mark the correct alternative in the following question:

The probability of guessing correctly at least 8 out of 10 answers of a true false type examination is:

  1. $\frac{7}{64}$

  2. $\frac{7}{128}$

  3. $\frac{45}{1024}$

  4. $\frac{7}{41}$

Answer

  1. $\frac{7}{128}$

Solution:

$\text{n}=10,\text{p = q}=\frac{1}{2}$

$\text{P(X}\geq8)=\text{P(8) + P(9) + P(10)}$

$\text{P(X}\geq8)=\text{ }^{10}\text{C}_8\big(\frac{1}{2}\big)^{10}+\text{ }^{10}\text{C}_{9}\big(\frac{1}{2}\big)^{10}+\text{ }^{10}\text{C}_{10}\big(\frac{1}{2}\big)^{10}$

$\text{P(X}\geq8)=\frac{45+10+1}{2^8}$

$\text{P(X}\geq8)=\frac{56}{256}=\frac{7}{128}$

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