Adding numbers

To do this task, use the + (plus sign) arithmetic operator.

For example, if you type the following formula in a cell:

=5+10

The cell displays the following result:

15




Source : http://office.microsoft.com/en-001/excel-help/add-numbers-HP003056115.aspx?CTT=5&origin=HP005200127#BMadd_numbers_in_a_cell