You write one formula, fill it down, and every row after the first is wrong. The reason is that references move as they are copied. A dollar sign is what stops them.
The formula
=B2*(1+$B$7) in column C
What each part does
B2
No dollar signs, so this moves down with the formula: B3, B4, and so on.
$B$7
Dollar signs pin the row and the column, so every row reads the same tax rate.
(1+...)
Adds the rate to one, which turns 0.2 into 1.2 and gives the price with tax.
How it works, and what to watch for
The general idea: a reference like B2 is relative. Copy it down a row and it becomes B3. That is what you want for the price on each row, and exactly what you do not want for a single rate sitting in one cell.
A dollar sign pins the part that follows it. $B$7 keeps both the column and the row, so every copy of the formula still reads B7.
You can pin one half only. $B7 keeps the column but lets the row move; B$7 does the opposite. These are useful when a formula is filled across as well as down.
Pressing F4 while the cursor is on a reference cycles through the four forms, which is quicker than typing the dollar signs by hand.
Questions
What does the dollar sign do in Excel?
It locks the part of the reference that comes after it, so that part does not change when the formula is copied to another cell.
Why do my numbers go wrong when I drag a formula down?
A reference that should have stayed still moved with the formula. Find the one that should be fixed and put dollar signs on it.
What is the shortcut?
Select the reference inside the formula and press F4. Each press moves through B7, $B$7, B$7 and $B7.
Set by your computer's language, not by the file. A frequent cause of a correct-looking formula being rejected.
Excel version
XLOOKUP, IFS, TEXTBEFORE and TEXTJOIN need Microsoft 365 or 2021; older versions return #NAME? instead. Choosing the older one switches every walkthrough to a formula that works there.
