As you can tell from the example, the exponent of 1/2 is the number of heads in a row.
a) p(6 heads in a row) = (1/2)^6 = 1/(2^6) = 1/64
b) p(12 heads in a row) = (1/2)^12 = 1/(2^12) = 1/4096
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<em>Additional comment</em>
The probability of a head is 1/2 because we generally are concerned with a "fair coin." That is defined as a coin in which each of the 2 possible outcomes has the same probability, 1/2. Similarly, a "fair number cube" has 6 faces, and the probability of each is defined to be the same as any other, 1/6. Loaded dice and unfair coins do sometimes show up in probability problems.
