Chapter 5 - Section 5.2 - Assess Your Understanding - Applying the Concepts - Page 279: 45a

P(uses social media) $=\frac{169}{265}\approx0.638$

The sample space: 530 adult Americans. So, N(S) = 530. According to the marginal distribution (see definition, page 235) of the first row: N(uses social media) = 338 P(uses social media) = $\frac{N(\text{use social media})}{N(S)}=\frac{338}{530}=\frac{169}{265}\approx0.638$