"Ex." is used to introduce exercises. Here's a look at how: "Please refer to ex. 4." The sentence instructs the reader to refer to an exercise later in the text, likely in an appendix. Rules for Using I.e., E.g., and Ex. in Writing. Now that we know what e.g., i.e., and ex. mean, let's look at how to use them correctly in writing. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.