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Excel has no way to paste only formulas. When you copy a formula and paste it to another cell all the original cell's formatting is copied as well.
But Excel has a way to copy only formats. This is done with the `Paint` function. So, this is what you do.
- Copy the formula down (in the F column), thereby destroying your borders.
- Select the cells in the E column whose format you want to paste to the F column
- Click on the 'Format Painter' in the Ribbon's Home tab
- Then immediately click on the first cell in the F column next to the first selected cell in the E column.
- All the formats of the selected cells will be copied to the F column leaving the formulas untouiched.
Perhaps a better way is to make use of the fact that you have groups of 3 rows followed by a subtotal row which should have another formula. So,
- Enter the formula in F3
- Copy/paste to F4:F5
- Restore the cell format in F4:F5 using the above technique
- Select F3:F6 and copy down to the bottom of your sheet.
- Bear in mind that you copy sets of 4 cells. Therefore when you select the destination you should also select sets of 4 cells before you hit Enter. If you select 19 or 21 rows instead of 20 the result will not be es expected.