Question

Below is the data of ages at the local swimming park on a certain day.

Answer 1

We know that
To determine the intervals you will need to find the lowest and highest numbers
lowest number------> 1
highest number-----> 35
(1 and 35). 

A general rule would be 5-7 intervals,
so
 I will choose 5.
Here are the intervals with the number of people that were in each:

[1-7] -----------> (9)
[8-14] ---------> (14)
[15-21] --------> (8)
[22-28] ------> (3)
[29-35]-------> (5)

The total of the frequencies was 39 people, and that is how many numbers there are.