Suppose it turned out that the 11.97 value in cell C4 for video rentals in February was incorrect. A rental of 3.99 was left out. To add 3.99 to 11.97, you would click in cell C4, type this formula into the cell, and then press ENTER:

**=11.97+3.99**

As the picture shows, when the value in cell C4 changes, Excel automatically updates the February total in cell C7 from 126.93 to 130.92. Excel can do this because the original formula =SUM(C3:C6) in cell C7 contains cell references.

If you had entered 11.97 and other specific values into a formula in cell C7, Excel would not be able to update the total. You'd have to change 11.97 to 15.96 not only in cell C4, but in the formula in cell C7 as well.

**Note** You can revise a formula in a selected cell by typing either in the cell or in the formula bar .