Explanation
In Tableau, you can aggregate measures or dimensions, though it is more common to aggregate measures.
Whenever you add a measure to your view, an aggregation is applied to that measure by default. The type of aggregation applied varies depending on the context of the view.
When you add a measure to the view, Tableau automatically aggregates its values. Sum, average, and median are common aggregations; for a complete list, see List of Predefined Aggregations in Tableau.
The current aggregation appears as part of the measure's name in the view. For example, Sales becomes SUM(Sales). Every measure has a default aggregation which is set by Tableau when you connect to a data source. You can view or change the default aggregation for a measure-see Set the Default Aggregation for a Measure.
You can change the aggregation for a measure in the view from its context menu:

Explanation
As the question mentions, we need to create a SET with the following conditions -> Choose only those customers whose Sales > 30,000
1) Right click on customer name --> Create --> Set

2) Let's Name the Set - Customer>30k ( you can name it anything you want :) ) Select USE ALL, and then move to the CONDITION TAB:

3) In the Condition Tab, Choose BY FIELD -> Select Sales -> Sum -> Greater than 30000 , and click OK

4) You should now have a new Set in the Data Pane as follows:

5) Drag this set to the rows shelf, and click on SHOW MEMBERS IN SET. Now drag Sales to the Column Shelf.

6) Click on the Show Mark Labels, and Sort ascending icons as shown:

7) Voila! We have our answer:
c

Explanation
A Pareto chart is a type of chart that contains both bars and a line graph, where individual values are represented in descending order by bars, and the ascending cumulative total is represented by the line.
On the primary axis, bars are used to show the raw quantities for each dimension member, sorted in descending order.
On the secondary axis, a line graph is used to show the cumulative total in percent format.

Explanation
The following are 2 correct ways to copy the worksheet visualisation as an image:

AND

Explanation
Not too tough. Follow along the steps:
* Drag Color to Filter and choose Red:
* 3) Now to display the percentage correctly, lets format it. Click on the In Stock % pill in the Row shelf, and select format:
* And your final view will look like :