1. The problem statement, all variables and given/known data In how many ways 4-lettered words that can be formed using the letters of the word "BOOKLET". The book solution solve it this way. The word contains 7 letters out of which there are 2 O's. So there are 6 letters. ∴ The number of 4 lettered words = 7P4 - 6P4 = 840 - 360 = 480 2. Relevant equations 3. The attempt at a solution What we need is to deduct the duplicate counting of 2 O's from the total permutation. So how does 6P4 solve this purpose. Instead of 6P2, isn't it should have been ( 4P2 * 5P2 ) / 2 i.e number of ways these 2 O's can be placed in 4 place * number of ways B,K,L,E,T can be placed in the remaining 2 places divide by 2. = (12 * 20)/2 =120 And the answer would be 840 - 120 = 720 and not 480.