
To add numbers in a cell, use the + (plus sign) arithmetic operator. For example, if you type the formula"=105+210" in a cell, the cell displays the result as "315".

Subtracting numbers in a cell is similar to adding numbers in cells. The + (plus sign) will be replaced by - (minus sign) in the formula. For example, if you type "=255-122" in a cell, the cell displays the result as 133

To add all contiguous numbers in a row or column, use AutoSum
icon available on the Standard toolbar.
To add noncontiguous numbers use SUM function as shown in formula examples given below :
Please See the above formulas in Formula bar and results in the cells in the images below:


To add numbers based on one condition, You can use the SUMIF function to create a total value for one range based on a value in another range, as in the following example:


Please see the formulas for computing the sales of Mike and Dennis in Formula bar and results thereof in the selected cells in above images.
Syntax for SUMIF function:
SUMIF(range,criteria,sum_range)
To add numbers based on multiple conditions, use the IF and SUM functions. Please look at the formula and the results in the example given below:

In the above example, the
Formula " =SUM(IF((B2:B11="South")*(C2:C11="Coffee"),D2:D11)) "
gives sum of Coffee sales in South region.
If you want to know the sum of Total sales in South and North regions, the formula will be
" =SUM(IF((B2:B11="South")+(B2:B11="North"),D2:D11)) "
Important :The formulas in the above example must be entered as array formulas (array formula: A formula that performs multiple calculations on one or more sets of values, and then returns either a single result or multiple results. Array formulas are enclosed between braces { } and are entered by pressing CTRL+SHIFT+ENTER.). To enter an array formula Press F2 and then press CTRL+SHIFT+ENTER. If the formula is not entered as an array formula, the error #VALUE! is returned.