Aptitude - Permutation and Combination - Discussion

I am not clear with this. They gave a condition that repetition is not allowed. when we use 10p4, (i.e. is permutation) it satisfy the given condition?

Please tell me that when we should use permutations and when we should combinations with an example?

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

! implies that it is a multiplication sign up to 1. It is neither a permutation nor a combination. For ex 4! implies that 4*3*2*1=24. is it clear guy?

Why permutation is used explain?

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

Please I want to know when to use combination and permutation.

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?

Which formula is used to solve this problem?

I have so much confusion at the answering problem,
"where we use the permutation formula and combination formula"?

Please anyone tell me.