Summary bar in Sheets iOS
Along the bottom of the Spreadsheet editor you'll find the Summary bar, which allows you to switch between the sheets that are currently present in your document.
If you select more than one cell that contains a number, you will automatically see the Sum of the selected cells on the right of the Summary bar. Tapping here will also let you see the Average, Count, Max, and Min of the selected cell range.
You can rearrange sheets by dragging them, and add new sheets using the 
Add button. Tapping on an existing sheet lets you Delete, Rename, and Hide/Unhide them.
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