In how many ways can the letters of the word ‘STRANGE’ be arranged so that
the vowels occupy only the odd places?
Given: the word is ‘STRANGE.’
To find: number of arrangements so that the vowels occupy only odd positions
Number of vowels in word ‘STRANGE’ = 2(E, A)
Number of consonants = 5(S, T, R, N, G)
Let vowels be denoted by V
Odd positions are 1, 3, 5 or 7
So, fix the position by Vowels like this:
Now, arrange these 2 vowels at 4 odd places
Formula used:
Number of arrangements of n things taken r at a time = P(n, r)
∴ Total number of arrangements of vowels
= the number of arrangements of 4 things taken 2 at a time
= P(4, 2)
= 4 × 3
= 12
The remaining 3 even places and 2 odd places can be occupied by 5 consonants
So, arrange these consonants at these places
Formula used:
Number of arrangements of n things taken all at a time = P(n, n)
∴ Total number of arrangements of consonants
= the number of arrangements of 5 things taken all at a time
= P(5, 5)
∵ 0! = 1}
= 5!
= 5 × 4 × 3 × 2 × 1
= 120
Hence, the number of arrangements so that the vowels occupy only odd positions = 12 × 120 = 1440