Explanation
Follow the steps to get the correct answer : January 2018

* We are talking about dates, so use the Time series sheet as follows:
* Next, the following should be your view and clearly, January 2018 is the lowest point:

Read more about dates:
https://interworks.com/blog/rcurtis/2017/01/30/tableau-deep-dive-dates-introduction-dates/

Explanation
Whenever you add a measure to your view, an aggregation is applied to that measure by default. This default is controlled by the Aggregate Measures setting in the Analysis menu.
If you decide you want to see all of the marks in the view at the most detailed level of granularity, you can disaggregate the view. Disaggregating your data means that Tableau will display a separate mark for every data value in every row of your data source.
To disaggregate all measures in the view:
Clear the Analysis >Aggregate Measures option. If it is already selected, click Aggregate Measures once to deselect it.

Explanation
According to the official Tableau documentation:
Tableau data source files have the .tds file extension. Data source files are shortcuts for quickly connecting to the original data that you use often. Data source files do not contain the actual data but rather the information necessary to connect to the actual data as well as any modifications you've made on top of the actual data such as changing default properties, creating calculated fields, adding groups, and so on. For more information, see Save Data Sources.

Explanation
From the official Tableau website:

Explanation
We need to use the concept of Groups for this question. Follow along:
1) First, Drag State to the Rows shelf, and Total Expenses to the Text Mark on the Marks Shelf:

2) Now, remove the SUM aggregation from Total Expenses, and add a quick table calculation -> Percent of total:

3) Next, Select the States of New York, California and Washington -> And then click the paperclip icon:

We can see the answer already : 23.386% in the view above (even before grouping!)
4) Finally, we get the following view and our answer: