You have a friend who claims to be psychic. You don't believe this so you test your friend by flipping a coin 20 times and having him predict whether each flip is heads or tails. If you are right, and your friend is NOT psychic, the the probability of guessing correctly on each flip is .5. Your friend correctly guesses 15 out of 20 flips.

Question

You have a friend who claims to be psychic.  You don't believe this so you test your friend by flipping a coin 20 times and having him predict whether each flip is heads or tails.  If you are right, and your friend is NOT psychic, the the probability of guessing correctly on each flip is .5.  Your friend correctly guesses 15 out of 20 flips.

What is the probability of your friend correctly guessing exactly 15 out of 20 flips if he is NOT really psychic?

Cassy · Answer

.015