Aptitude - Permutation and Combination - Discussion

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

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

Whereas in
Combination - ordering is not considered.

How to know whether to use permutation or combination?

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

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

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

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

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

It can also be done in another way.

Number of ways of selecting 4 letters from 10 letters=10C4
=(10*9*8*7)/4!

Number of ways of arranging these 4 letters =4P4=4!

So, total number of words formed= [(10*9*8*7)/4!]*4! = 10*9*8*7 = 5040

Am I correct?

It is a four letter word ---- out of 10 repeatition is not allowed so first 10 we have 1 then out of nine wehave 1then out of 8 we have 1 then out of 7 we have 1 so 10*9*8*7=ans