In how many different ways can the letters of the word MACHINE be arranged so that the vowels may occupy only the odd positions?
Options:
A .  210
B .  576
C .  144
D .  1728
E .  3456
Answer: Option B There are 7 letters in the given word, out of which there are 3 vowels and 4 consonants. Let us mark the positions to be filled up as follows: $$\left( {\mathop {}\limits^1 } \right)\left( {\mathop {}\limits^2 } \right)\left( {\mathop {}\limits^3 } \right)\left( {\mathop {}\limits^4 } \right)\left( {\mathop {}\limits^5 } \right)\left( {\mathop {}\limits^6 } \right)\left( {\mathop {}\limits^7 } \right)$$ Now, 3 vowels can placed at any of the three places out of four marked 1, 3, 5, 7 Number of ways of arranging the vowels $$\eqalign{ & = {}^4{P_3} \cr & = \left( {4 \times 3 \times 2} \right) \cr & = 24 \cr} $$ 4 consonants at the remaining 4 positions may be arranged in $${}^4{P_4} = 4! = $$ 24 ways Required number of ways = (24 × 24) = 576
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