Chapter 7 - Section 7.7 - Basics of Probability - 7.7 Exercises - Page 689: 2

The sample space is $S=${$HH; HT; TH; TT$}

During coin flip experiments we use the letter $H$ for heads and letter $T$ for tails. The sample space is a set that consists of all the possible results of the experiment. Here, each of the coins can be heads or tails up after landing. Therefore there will be $2\times2=4$ possible outcomes. The sample space is $S=${$HH; HT; TH; TT$}