Answer:
There exist 2 cases to consider :
Case 1 - The two O's are not identical
Then it is equivalent to asking in how many different ways can 5 different units be arranged in a row.
This is simply
5
!
=
120
different ways.
Case 2 - The two O's are identical
This is then equivalent to asking in how many different ways can 5 items be arranged in a row, if 2 of the 5 items are identical.
By the addition principle, there are 10 possible ways to place the 2 identical objects in the 5 available slots in the row.
Now for each of these 10 different arrangements of the identical items, the other 3 non-identical items may be placed and arranged in
3
≠
6
different ways.
So in total, there are then
6
×
10
=
60
different ways.
Note that since the 2 identical objects can themselves be arranged in
2
!
different ways, we don't include this in the total since these arrangements are considered identical to one of the others already.