In this first example, we will add cells A1 (25) and A3 (45).
 The formula to this is "=sum(first_cell, second_cell)" (quotation marks not used)

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In this screenshot we can see the result of adding cells A1 and A3

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 In the second example we will add cells A1 through A4.
The formula to do this is "=sum(first_cell:last_cell)" (quotation marks not used)

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In this screenshot we can see the result of adding cells A1 through A4

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 In this example we will start with cell A1 (160) and subtract cells A2, A3, and A4.
 The formula to do this is "=(A1-A2-A3-A4)" (quotation marks not used)

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In this screenshot we can see the result of subtracting cells A2, A3 & A4 from cell A1.

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