Bunuel
How many five letter words (with or without meaning) can be formed such that the letters appearing in the odd positions are taken from the unrepeated letters of the word MATHEMATICS whereas the letters which occupy even places are taken from amongst the repeated letters of the same?
A. 3600
B. 540
C. 480
D. 420
E. 360
MATHEMATICS
Repeated set of letters = 6 {M, A, T, M, A, T}.
Unrepeated set of letters = 5 {H, E, I, C, S}.
letters appearing in the odd positions are taken from the unrepeated letters
--> Number of possible ways of selecting = \(5c_3 = 10\)
letters which occupy even places are taken from amongst the repeated letters of the same = 2 letters
--> Number of 2 letters possible = {(M, M), (A, A), (T, T), (M, A), (A, T), (T, M)} = \(6\)
Case 1: When even places are of same digits.
Number of possible arrangements = \(5c_3*3!*3*1\) = \(180\) [\(5c_3*3!\) = Odd places; \(3*1\) = Even places]
Case 2: When even places are of different digits.
Number of possible arrangements = \(5c_3*3!*3*2! = 360\) [\(5c_3*3!\) = Odd places; \(3*2!\) = Even places]
--> Total number of arrangements possible = \(180 + 360 = 540\)
Option B