A formula cannot change the contents of a different cell to itself.
Here's a method that might fill the bill:
I am not sure why you would do so, but here is an option: =B3&" & "&C3
It is a formula that the user creates themselves, instead of using the built-in functions.
Not in Excel.
The Clipboard.
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
In versions up to Excel 2003, it is on the Data menu. From version 2007 onwards, you can find it on the Insert tab on the ribbon.
what-if analysis or sensitivity analysis Its What-if Analysis
You can enter them by just typing them in with the keyboard. You can also use the Insert Function facility.
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.
If the formula is something like AVERAGE, SUM, etc., then it will copy it to another cell. However, when using the sizing handle after just entering the formula alone, Excel will automatically fill in the answer. The formula is copied along with it, though. However, if the formula is in reference to the content of another cell, this will not happen. The formula changes if the formula uses the content of another cell. To keep it exactly the same to copy it down, you have to make the cell number in the formula an absolute value so it will not change.