So, if you don't care about the answer being inserted into the document automatically and you just want to know the answer to a simple equation already in a Word document, I think this it the best out-of-the-box, offline, no equation box required, no addins required option: https://www.howtogeek.com/267279/how-to-perform-simple-calculations-in-microsoft-word/
When you need to do a quick calculation, normally you would think to use the Windows calculator. However, if you’re working in Microsoft Word, you can calculate simple equations typed into your document using Word’s not-so-obvious Calculate command.
To use the Calculate command, we need to add it to the Quick Access Toolbar. To do this, click the down arrow button on the right side of the Quick Access Toolbar and select “More Commands” from the drop-down menu.
Select “All Commands” from the “Choose commands from” drop-down list.
In the list of commands on the left, scroll down to the “Calculate” command, select it, and then click “Add”.
The Calculate command is added to the list on the right. Click “OK” to accept the change.
Now you can type in and then select a simple equation (do not select the equals sign) in your Word document and click the “Formula” button. For some reason, the button is not called Calculate.
You do have to select/highlight the text you want it to evaluate, if you miss part of the selection then it doesn't get rolled into the answer. That wasn't quite explicitly clear in the instructions.
If anyone knows the background of this magical button, or knows how to get it through a different means other than the quick access toolbar addition, that would be good to know.