Why this matters
An accounts assistant who types totals by hand will eventually make a mistake, and nobody will know where. A formula shows its working: anyone can click the cell and see how the answer was calculated. Employers want reports they can trust, and formulas are how you earn that trust.
Writing a formula
A formula starts with =, then uses cell addresses and operators:
+add,-subtract,*multiply,/divide,^power- Brackets work as in school maths:
=(B2+C2)*D2adds first, then multiplies.
Use cell addresses instead of typing numbers. =B2*C2 is better than =5*120, because when B2 changes, the answer changes too.
Copying formulas: relative references
Here is the clever part. If you write =B2*C2 in D2 and copy it down to D3, Excel changes it to automatically. The references are : they mean "the cell two to the left times the cell one to the left", whatever row you are on. That is why you can fill a formula down a thousand rows in one move by dragging the fill handle (the small square at the corner of the selected cell) or double clicking it on a computer.