Annie will toss a fair coin three times. what is the probability that all 3 tosses will result in heads?

For a fair coin:
There are two possibilities, Head H or Tail T.
Total sample space = 2

Probability of Head = 1/2

Probability of Tail = 1/2

For all three tosses to be Head = (HHH)

P(HHH) = P(H) * P(H) * P(H)

               = (1/2) * (1/2) * (1/2) = 1/8

Probability of 3 tosses been head = 1/8