How to do addition in Excel
See the image, showing the addition of 10, 20 and 30.
Choose the cell where you want the result to appear as cell C6 in this example. Then type = sign in the cell, now select first cell C3 then type "+" sign then select second cell C4, again type + and select third cell C5. Now press the enter key, you will get the sum of these three numbers.
The red arrow is showing the result of addition. See the image: