Paragraph: This set contains additional number series questions,...

Paragraph: This set contains additional number series questions, some of which are in Roman numerals. These items differ from Sets 1,2 , and 3 because they ask you to find the number that fits somewhere into the middle of the series. Some of the items involve both numbers and letters; for these questions, look for a number series and a letter series. Question: Look at this series: $\frac{1}{9}, \frac{1}{3}$, 1, $\_\_\_\_, 9,$\ldots What number should fill the blank?