How can I keep a tally in Excel by going back to the same cell and
entering slash marks without deleting the previous slash marks. This
is for a test and I need to keep a total of how many answered a
specific question.

This isn't the method you asked for, but it might help.

First the preparation:
- Find an out-of-the-way cell and enter the number 1 there.
- Use "Edit Copy" to copy the 1 to the clipboard.
- Select another out-of-the-way cell and use:
Edit Paste special Operation "Add"

Now for each time you want to increment a tally:
- Select the cell
- Hit the F4 key

So to tally a whole bunch of totals, go:
Select F4 Select F4 Select F4 Select F4 ...

(If you do anything in between, you have to do the preparation again.)