Originally Posted by
Rapha I don't know. But I guess it's kinda helpful knowing what the probability is.
10 questions, 3 (questions correct answered) correct answers
=> $\displaystyle \begin{pmatrix} 10 \\ 3 \end{pmatrix}$
5 choices per question;
probability for guessing the right answer is therefor $\displaystyle \frac{1}{5} = 0.2;$
probability for guessing the wrong answer is therefor $\displaystyle \frac{4}{5} = 0.8.$
=> probability("three right answers") = $\displaystyle \begin{pmatrix} 10 \\ 3 \end{pmatrix}*0.2^3*0.8^7 \approx 0.20$
the probability of guessing exactly 3 correct answers out of 10 is 20% (=0.2)