NCERT Solutions
Class 11 Maths
Permutations and Combinations

Ex.7.3 Q.11
In how many ways can the letters of the word PERMUTATIONS be arranged if the
(1) words start with P and end with S,
(2) vowels are all together,
(3) there are always 4 letters between P and S?
In the word PERMUTATIONS, there are 2 Ts and all the other letters appear only once.
(1) If P and S are fixed at the extreme ends (P at the left end and S at the right end), then 10 letters are left.
Hence, in this case, required number of arrangements = 10! ÷ 2! = 1814400
(2) There are 5 vowels in the given word, each appearing only once.
Since they have to always occur together, they are treated as a single object for the time being.
This single object together with the remaining 7 objects will account for 8 objects.
These 8 objects in which there are 2 T’s can be arranged in 8! ÷ 2! ways.
Corresponding to each of these arrangements, the 5 different vowels can be arranged in 5! ways.
Therefore, by multiplication principle, required number of arrangements in this case = 8! ÷ 2! × 5 = 2419200
(3) The letters have to be arranged in such a way that there are always 4 letters between P and S.
Therefore, in a way, the places of P and S are fixed. The remaining 10 letters in which there are 2 T’s can be arranged in 10! ÷ 2! ways.
Also, the letters P and S can be placed such that there are 4 letters between them in 2 × 7 = 14 ways.
Therefore, by multiplication principle, required number of arrangements in this case = 10! ÷ 2! × 14 = 25401600