Step 1: Open the file, click 'Edit', convert it to 'Edit Mode'. Like the fillowing:
Setp 2: Click the button ‘Tools’.
Step 3: In "Tools", there is an ’AutoSum’ option below ‘Data’ Tab.
Step 4: Click ’AutoSum’ to find the option 'Sum'.
Step 5: Set the range of cell where ought to be sumed. You can drag the cells or input the cell range.
Step 6: At last, click enter to confirm.
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