GMAT Focus Edition Practice Test

Three independent coin flips yield 2^3 = 8 equally likely sequences. To get exactly three heads, you need heads on every flip, which is just one of those eight outcomes. So the probability is 1 out of 8, i.e., 1/8. This can also be seen by multiplying the chance of a head on each flip: (1/2) × (1/2) × (1/2) = 1/8. The other numbers would correspond to having fewer than three heads, not three.