:3. Solution.  Keep track of of how the difference changes as
the tosses are made.  If a head comes up, the difference
increases by 2.  If a tails comes up, the difference goes down by
1, that is "increases" by -1.  So the numbers on the tickets in
the box have to be 2 and -1.  The only possible choice is the box
with one 2 and one -1. <p>