SUM using names

You can use the names typed at the top of columns or side of rows in calculations
simply by typing the name into the formula.

Try this example:
Go to cell C16 and then enter the formula =SUM(jan)
The result will show.
This formula can be copied to D16 and E16, and the names change to Feb and Mar.

If it does not work !
The feature may have been switched off on your computer.
You can switch it on by using Tools, Options, Calculation, Accept Labels in Formula.