How to Work with Formulas in Numbers Spreadsheets
Formulas calculate values based on the contents of cells you specify in your Numbers spreadsheet. Numbers is the spreadsheet application that is part of the iWork suite of products on your MacBook.
For example, if you designate cell A1 (the cell in column A at row 1) to hold your yearly salary and cell B1 to hold the number 12, you can divide the contents of cell A1 by cell B1 (to calculate your monthly salary) by typing this formula into any other cell:
=A1/B1
By the way, formulas in Numbers always start with an equal sign (=).
So why not use a calculator? Maybe you want to calculate your weekly salary. Rather than grab a pencil and paper, you can simply change the contents of cell B1 to 52, and — boom! — the spreadsheet is updated to display your weekly salary.
That’s a simple example, of course, but it demonstrates the basis of using formulas (and the reason that spreadsheets are often used to predict trends and forecast budgets). It’s the “what if?” tool of choice for everyone who works with numeric data.
To add a simple formula within your spreadsheet, follow these steps:

Select the cell that will hold the result of your calculation.

Click inside the Formula Box and type = (the equal sign).
The Formula Box appears to the right of the Sheets heading, directly under the Button bar. Note that the Format Bar changes to show a set of formula controls (a.k.a. the Formula Bar).

Click the Function Browser button, which bears the fx label. (It appears next to the red Cancel button on the Formula Bar.)

In the window that appears, click the desired formula and click Insert to add it to the Formula Box.

Click an argument button in the formula and click the cell that contains the corresponding data.
Numbers automatically adds the cell you indicated to the formula. Repeat this for each argument in the formula.

After you finish, click the Accept button to add the formula to the cell.
That’s it! Your formula is now ready to work behind the scenes, doing math for you so that the correct numbers appear in the cell you specified.
To display all the formulas that you’ve added to a sheet, click the Formula List button in the toolbar.