How to protect the cell's formula not be deleted

[Man Utd]

I have formula in cells, how can I protect this formula from being deleted
when the user select this cell and delete the content.

eg.
in cell C10, the formula is =A10 * 2

If the A10 has value of 10, then C10 will be 20.
If the user select A10 and delete the content 20, then the formula will also
be deleted.
How do I prevent that ?

[mangesh_yadav]

Select all cells
Format > cells > protection > Uncheck Locked
Select the cell with formula
Format > cells > protection > Check Locked
Tools > Protection > Protect sheet (password is optional)

Mangesh