You can sum cells across multiple sheets in Excel. This article presents steps for Adding single-cell values or Adding values in the range of cells.

Microsoft Excel PivotTables turn data into meaningful information, but it's difficult to add grand total rows. Read on to learn how.

So this is how you can enable multiple selections in a drop-down in Google Sheets. While this is not available as an in-built feature, you can easily do this with some Google Apps Script magic.