I have for years inserted simple math formulas into Excel with no problem.  I had to evaluate three bid proposals for re-piping our house with cooper or PEX.  I set up my Mac with Numbers and proceeded to enter data.  I suddenly got an error message after entering a rather simple math formula.

The operator “-” expects a number, date, or duration, but cell D3 contains a string.’

The problem was created when the formula used the header cells instead of the data cells.

In the image above, you can see that while I had selected the cell on the second line to enter the formula, the cells that were part of the formula were actually the header.  I really do not know how this happened.  As soon as I entered any alpha character (label) into the header, I got a formula error.  To solve this problem I made darn sure to enter the formula into the single data cell and not include any header cells as part of the formula.

In the image above, you can see that the formula, when shown on the computer screen, highlights data cells only and no header cells.

I then discovered that I only needed to copy the formula cell, on line 2, that I had just created successfully and paste it down the spread sheet where I wanted the formula to be replicated.