You toss a coin until you get a total of k heads. what is the expected number of coin tosses

2k. The probability of getting heads on a coin toss is (roughly) 50%, so if you flipped, just as an example, 20 coins, you would expect to get around ten heads, or 20/2. The same basic concept can be applied to a variable, where the number of heads, k, is half the number of tosses, and k*2 is 2k.