In-line R expression returns incorrect value

The problem is that in your header, you are "previewing" your file, which does not actually run your code from scratch. You have to knit it to HTML to have it run so that your in-line code is correct.

Problem Header

---
title: "R Notebook"
output: html_notebook
---

Solution Header

---
title: "R Notebook"
output: 
    html_document: default
    html_notebook: default
---

Other Notes

The previous solution has two problems. First, from the RMarkdown documentation, "Inline expressions do not take knitr options" (see end of http://rmarkdown.rstudio.com/lesson-4.html)

Second, the previous answer's YAML is not formatted properly forcing RStudio to actually knit the file. The proper formatting would generate the same problem you are having

---
title: "R Notebook"
output: 
    html_notebook: default
---