Whenever I enter a decimal number in a cell and try to use it in a formula, I
get an error. For example, if I have cell A1 as 15 and cell A2 as 1.1 and
use the formula =A1*A2 or even something like =A1*1.1, I get an invalid
reference message. When I perform error checking help, it tells me that the
value is the wrong data type.

So, I'm guessing Excel has somehow changed the default for decimal numbers
to text, but if I go to Format--Cells for the cell in question, it tells me
that the format is actually a number with 1 decimal place (but the error
remains).

I can't figure out how to fix it. This is happening even after rebooting
and opening a new file.