Aptitude - Permutation and Combination - Discussion

The formula we hv usd here is = n!/(P1!)(P2!)...(Pr!)
Here n=10(no. of letters) and P1,P2.. are the different letters of the word.
Then the no. of permutation of these 10 letters is
= 10!/(1!)(1!)..upto 10th letter [(1!)...upto 10th letter used bcoz each letter occured only once]
= 10*9*8*7 [bcoz we hv to make 4 letter words]
= 5040

Why you have used permutation here? please tell me the repetions of 4 letters concept?

I'm also wondering why we are using permutations here. According to the question no order is required.

Logarithms===> total= 10 alphabets.. okay
we have to form 4-letter words, without repeating the letters.
_ _ _ _
we have place for 4 alphabets
In 1st blank, out of 10, any1 of it can come,so we have 10 choices--->10
In 2nd blank, now,9 letters are left, out of 9 u can choose one---->10*9
In 3rd blank, now 8 left, i.e.--->10*9*8
In 4th blank, any remaining of 7 letters can come---> 10*9*8*7
= 5040

Why we used permutations here and why not combinations.
Please explain clearly when we use npr and when we use ncr.

How to know whether to use permutation or combination?

Permutation = when ordering is considered.
(means here to form a word ordering is important).

Whereas in
Combination - ordering is not considered.

So it means when repetitions not allowed it is an ordering condition ! ?

Yes, when repetitions not allowed it is an ordering condition.

How can I identify the given question from permutation or combination?