If you have formatted the cells to display with 2 decimal places, this did not affect the underlying numbers. The actual numbers still have all their decimal places, you just don't see them. Use the ROUND function if you want them to actually have two decimal places. Then they'll add up to what you want.
Actually, the addition gave you the 'right' result, or more accurately, a closer approximation of the correct sum.
Badunit has given you the method to obtain the less accurate, but 'expected' result.
You will find ROUND in the same place as you can find every function supported in Numbers—in the Function Browser. Click the Function button (ellow, and labeled with an ƒ ) and choose Show Function Browser.
In the browser, type 'rou' or as much more of the name to bring the function name into view. Select it and click Insert to insert it into the cell containing the insertion point.
Sorry i am new to numbers. Thank you for your help.
I did what you mentioned and my cell gives me error with a red triangle when I use the ROUND function.
It says "you must specify a value for all the required arguments".
The cell I am rounding is a sum of two other cells. When I round the cell converts my SUM formula to
= ROUND (num-to-round,digits).
Best advice I can give to someone new to numbers (or new to spreadsheets in general) is to download and read at least the first four chapters of the Numbers '09 User Guide.
If you're going to write formulas, you'll also want to have the iWork Formulas and Functions User Guide, which lists and describes all the functions supported in numbers, and gives at least one example of using each in a table.
Both guides are available for download via the Help menu in Numbers.
Regarding your current issue:
Put your SUM() formula in the space labeled 'num-to-round' and the number of decimal places you want to have in the result in the space labeled 'digits'.
Here's and example:
The formula in D2 is shown above the table.
The three other cells in the column calculate the same sum, but round it to the nearer tenth, whole number, and hundred respectively. Formulas listed below:
You need to use the ROUND function on the cells you are going to sum up. In your example, that would be the cells with the 10.10's in them. You need them to actually be 10.10 before you sum them up, not 10.0967. If you only round the sum, you will get the same result you are already getting, 30.29.
Your formula will look like
=ROUND(the formula that was in the cell before but without the equals sign, 2)