Correct Answer - Option 3 : 2 ×
24P
5 × 20!
Concept:
Combinations: The number of ways in which r distinct objects can be selected simultaneously from a group of n distinct objects, is:
nCr = \(\rm \frac {n!}{r!(n-r)!}\).
Permutations: The number of ways in which r objects can be arranged in n places (without repetition) is:
nPr = \(\rm \frac{n!}{(n - r)!}\).
Calculation:
There are 26 letters in the English alphabet. If we separate the group (a, some 5 letters, b), we will be left with 19 more letters.
These 20 objects (1 group + 19 letters) can be arranged among themselves in 20! ways.
Since either a or b can be at the beginning or the end of the group of 7 letters (a, some 5 letters, b), the number of possible arrangements of the group will be 2 × (1P1 × 5P5 × 1P1) = 2 × 5!.
Also, each group of 5 letters can be selected from the remaining 24 letters (except a and b) in 24C5 ways.
Required total number of ways = (2 × 5! × 24C5) × 20!
= 2 × 24P5 × 20!.