Function Method

Move again to cell C9 and Delete the formula by tapping the Delete key.
Now type in the following: =SUM(
[This tells Excel that we are going to sum some numbers in a RANGE which will follow the =SUM(]
Notice – a new feature in Excel 2007: As you are typing SUM in cell C9 that a pop-up menu appears under the cell. What you see are mathematical functions. One of these is SUM. As you become more comfortable with numbers in Excel, you can select the functions you need without typing in the entire function. We’ll get into this a bit more later.
There are two ways to put in this range:
Arrow Key and Anchor Method: With the keyboard arrow keys, move the cursor to cell C6. As you move you will notice that the cell where the cursor is located appears after the =SUM(. When you get to C6 tap the . (Period) Key. This is called an ANCHOR and holds one end of the RANGE in place. You will notice that a C6:C6 appears in the formula area under the button bar. This is a one cell range. Now move, with the arrow keys, to cell C8. See how cells C6, C7 and C8 are highlighted. This indicates the Range is C6:C8. Excel assumes, logically, that these are the numbers you want to add. Now tap Enter. The numbers still add, but now the formula reads =SUM(C6:C8) instead of =C6+C7+C8 like it did before.

Mouse Method: Move again to cell C9. Delete the formula in cell C9 by tapping the Delete key. Type in =SUM( as you did before. Point to Cell C6 – with your mouse cursor. Click and hold down the left mouse button and move/drag the cursor down to Cell C8 (Cells C6, C7 and C8 should be highlighted) – take your finger off the left mouse button. Tap Enter.
This =SUM Function is a great way to add a lot of numbers, or a block/range of numbers. By simply anchoring, and using page downs, or using the mouse, you can highlight lots and lots of numbers to add quickly. However, since it only sums you can't do subtraction, etc.
Point to cell C9 again. Tap the Delete key to remove the formula currently in cell C9. This is a really important DELETE, since what we’ll explain below won’t work correctly if you do not delete the formula in cell C9.
Functions
There are a number of formulas built into Excel, like Sum. These formulas are called Functions.
Another new feature of Excel 2007 – Tabs/Ribbons. Look at the top of your Excel screen and click on the Formulas Tab. The Formulas Ribbon will display.
On the left of the Formulas Tab/Ribbon is an Insert Function button. Click the Insert Function button.
The Insert Function menu screen will appear (image at right).
Let’s work with the Insert Function menu screen. Click the small down arrow to the right of Or select a category: (see arrow at left).
In the drop down menu that appears you can see that there are all kinds of formulas (functions) that come with Excel spreadsheet (e.g. statistical, mathematical, financial, etc.). Instead of having to go to math, financial, or statistical tables in a book, you can enter data from your spreadsheet into the formulas and