How do you write formulas for Excel?

There is no simple answer to that in one way, in that there is no limit to the amount of formulas you could come up with. On a simpler level, the first thing to know is that all formulas must start with the equals sign. There are hundreds of functions you can use and you can do very complex formulas and very simple ones. Here are a few instructions to create a few simple ones.

Put any number you want into cell A1. Then put any number you want into cell A2. Then in cell A3 type:

=A1+A2

That will add the two numbers you entered and put the total in A3. If you change the two numbers, the formula will always add them up and give you the correct answer. If you change the + to a - or / or * you will get a subtraction formula, a division formula or a multiplication formula.

The formula in A3 cannot mention A3 in it. That is another rule of formulas. A formula in a cell can never refer directly to the cell itself, otherwise you get what we call a circular error.

Excel has lots of functions, which are sort of pre-designed formulas that you can do to do things quicker. This time put in numbers into all the cells from B1 down to B10. Then in B11 type:

=Sum(B1:B10)

It will add up all the values from B1 through to B10 and put the total in B11. If you change the word Sum to Average and leave the rest of the formula as it is, it will give you an average of all the figures from B1 to B10. Changing it to Max will give you the highest number in those cells and changing it to Min will give you the lowest number.
You can, when typing in certain cells, go to the text box at the top and enter certain codes for the type of formula you want. You can also include other squares (like, A1 added by A4 equals). There are also buttons in the menu or ribbons at the top of the program that can do formula actions to cells that are highlighted.