Question:
In how many different ways can 20 different pearls be arranged to form a necklace?
Solution:
We know that necklace in the form of a circle, So we need to arrange 20 pearls in Circle
20 pearls can be arranged by $19 !$
Now each pearl have the same neighbour in the clockwise and anticlockwise arrangement
Total number of arrangement are $(19 !) / 2$