Final answer:
Robert's coin-flipping experiment can result in four different outcomes: HH, HT, TH, and TT. These outcomes form the sample space and each is equally probable with a fair coin.
Step-by-step explanation:
When Robert flips a coin twice, he is performing a simple probability experiment that can have four different combinations of outcomes. The possible outcomes for each flip are heads (H) or tails (T), making the sample space for two flips {HH, HT, TH, TT}. Each outcome listed is unique, where HT represents the first flip being heads and the second flip being tails, while TH represents the first flip being tails and the second flip being heads. Because we assume a fair coin is used, each of these outcomes is equally likely.