No, not with a formula. What you can do is use the Filter facility to hide records, which are always in rows. Using an Advanced Filter, you can extract data from an overall set.
It is a formula that the user creates themselves, instead of using the built-in functions.
You can click on it in the Formula Bar near the top of screen, or you can press the F2 key.
It is a function
It is used to copy the formula down or across.
Not entirely, but you can use a mouse to select ranges you include in a formula.
You would first need to know how the fat in milk is calculated and have a formula for it. You would need to know what values are needed to do it. Then you would need to get the values you need and enter them into cells in the spreadsheet. Using these cell references to build your formula, you could do the calculation.
By using absolute or mixed references. See the related question below.
In Excel it allows you to enter formulas using the mouse, by clicking on cells as you type a formula, instead on typing the cell references.
Under the formulas tab, there is an option called "Show Formulas" in Formula Auditing. This applies for Excel 2010 (what I am using) and should apply for Excel 2003 and 2007 as well. For older options (or if there is no such option in Excel 2003/2007, search for something similar).