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,
You cannot 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.
How do you pay for your 7 nights at the hotel?