Aptitude - Permutation and Combination - Discussion

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

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.

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

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

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

If position of word matter (no repetition allowed) Then we have to use permutation.

And if no position matter we have to use combination.

When we form LOGA and AGOL it is permutation but in combination we don't write like r, s is similar to s, r -in combination, not in permutation. So in above question we are not allowed to double the letter but are allowed to use the same letter in different words. .