The Seven Rings
You arrive at a hotel and have 3 sets of golden rings. The first set of
rings has 4 rings, the second set has 2 rings and the third only has one
ring. You cannot take these sets of rings apart, exchange them for a
different form of currency, and the hotel clerk has no change. You want
to stay at the hotel for 7 nights, and you have to pay one gold ring for
each night that you stay. You cannot pay in advance, or all at once at
the end of your stay.