A fair coin is flipped 4 times, what is the probability of obtaining at least one head in the first two tosses?
closed as off-topic by Sycorax, Matthew Drury, Juho Kokkala, Tim♦, kjetil b halvorsen Feb 28 '17 at 12:56
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Assuming the coin is fair, the first two tosses can be TT, TH, HT, or HH with equal probability for each pair. The last three pairs (three of the four) match your desired first two flips, and the last two flips do not matter because each pair can have the same last two flips. So the probability of obtaining at least one head in the first two of four tosses is $3/4$.