## Explanation

This formula is a good example of how structured references can make working with data in Excel much easier. At the core, this is what we're doing:

```
=AVERAGE(first:last)
```

where "first" is a reference to the first cell to include in the average and "last" is a reference to the last cell to include. The result is a range that includes the N cells to average.

To get the first cell in the range, we use INDEX like this:

```
INDEX(Table1[Sales],ROWS(Table1)-(F4-1))
```

The array is the entire Sales column, and row number worked by subtracting (n-1) from total rows.

In the example, F4 contains 3, so the row number is 10-(3-1) = 8. With a row number of 8, INDEX returns C12.

To get the last cell we use INDEX again like this:

```
INDEX(Table1[Sales],ROWS(Table1))
```

There are 10 rows in the table, so INDEX returns C14.

The AVERAGE function then returns the average of C12:C14, which is $78.33.