Problem: There's a student who has to answer $4$ multiple choice questions (with $5$ possible answers, from which only $1$ is correct). We want to know the probability that the student gets $3$ answers right by answering at random?
Answer: $2,56\%$
I know we have to use Bernoulli's principle, but I have no idea how to use it and how to get an answer from it.
He repeats trial 4 times. To success one time is $p={1\over 5}$ and to success 3 times out of 4 times is $${4\choose 3}p^3(1-p) = 4\cdot {1\over 5^3}\cdot {4\over 5} ={16\over 625} = 0.0256$$