Two identical 1.5 volt batteries have the positive terminal of one
battery connected to the negative terminal of the other.  How does the
voltage of the combination compare with the voltage of a single
battery? Explain

Hi Sparkle13, 

The circuit you describe has the batteries arranged in series
(negative terminal to positive terminal). The voltage of the battery
combination is the sum of the voltage of the two batteries, so 3v. If
a third 1.5v battery was wired in the same way the voltage would be
4.5v.

If the batteries were wired in parallel (negative terminal to
negative, positive to positive), the voltage would remain at 1.5v. The
capacity (in amp hours) would double.

You may find it useful to read this short article at zbattery.com
called "Connecting Your Batteries in Series or Parallel":
http://www.zbattery.com/seriesparallel.html

I hope this helps, 

Hibiscus

Rant on:
I should point out that Google explicitly discourages answering
homework questions. If you want to help: give a hint, or point to a
source of information. Simply giving the answer doesn't help anyone in
the long term
Rant off