If you want to add the numbers in cells A1 to A3, type the numbers in cells A1 to A3, then select the blank cell A4 and type =A1+A2+A3 and hit enter. You can use this technique to add as many cell as you like. But if want a still easier way to do this, then select the cell where you want the sum of cells to be displayed and click on the autosum button and press enter.
It is a very time consuming practice to enter all the months or days or any other sequential data in adjacent rows or columns one by one. To reduce this effort, just follow these steps.
Type the first entry
Place your cursor at the bottom right corner of the cell
When the cursor transforms into a thin black + sign, press the left mouse key and drag it to across the rows or columns according to your requirement.