176.Two Balls, Two Whites
★★★conditional probabilityJane Street
A bag holds two balls, each independently black or white with probability one half. You draw a ball, see white, replace it, draw again, and see white again. What is the probability that the next draw is also white?
Sign in to submit an answer and track it toward your stats.
Sign inHint
Update the composition first. Two white draws are impossible from an all-black bag and four times as likely from an all-white bag as from a mixed one.
Want timed drills, mental math, and a market-making game too? Try the full practice suite →