What is the mode of the data set: 1, 2, 2, 3, 4, 5, 5?

To determine the mode of a data set, you need to identify the number that appears most frequently. In the given set: 1, 2, 2, 3, 4, 5, 5, the number 2 appears twice, and the number 5 also appears twice. All other numbers occur only once.

Since both 2 and 5 appear with the highest frequency in the data set, they are both considered modes. This makes it a bimodal data set, where there are two modes: 2 and 5. Thus, acknowledging both values as modes accurately represents the data's characteristics.