In: Advanced Math
How many ways can a gardener plant five different species of shrubs in a circle? What is the answer if two of the shrubs are the same? What is the answer if all the shrubs are identical?
Let
be the different species of shrubs to be planted in five different
places
on a circle.
has five choices to get planted.
has four choices,
has three,
has two and
has only one choice to get planted. So, total number of structures
(with repetition) we get on a circle is
. But here, every structure is repeated
times. So the total number of different structures i.e. number of
ways of planting these shrubs on a circle is
.
Now, suppose two shrubs, say
and
, where
, are of the same species. So any two structures with
and
interchanging their places will be the same. Therefore Number of
ways in this case is
.
When all the shrubs are the same, naturally there is only one way to plant them on a circle.