JEE MAIN - Mathematics (2023 - 13th April Evening Shift - No. 3)
All words, with or without meaning, are made using all the letters of the word MONDAY. These words are written as in a dictionary with serial numbers. The serial number of the word MONDAY is :
324
328
326
327
Explanation
The letters of "MONDAY" arranged in alphabetical order are: A, D, M, N, O, Y.
- Fix A as the first letter, we can arrange the remaining 5 letters in 5! ways = 120 ways.
- Fix D as the first letter, we can arrange the remaining 5 letters in 5! ways = 120 ways.
So far, we have 120 (for A) + 120 (for D) = 240 words.
Now, let's proceed with words starting with M:
- Fix MA as the first two letters, we can arrange the remaining 4 letters in 4! ways = 24 ways.
- Fix MD as the first two letters, we can arrange the remaining 4 letters in 4! ways = 24 ways.
- Fix MN as the first two letters, we can arrange the remaining 4 letters in 4! ways = 24 ways.
Now, we have 240 (for A and D) + 24 (for MA) + 24 (for MD) + 24 (for MN) = 312 words.
After MN, we consider words that start with MO:
- Fix MOA as the first three letters, we can arrange the remaining 3 letters in 3! ways = 6 ways.
- Fix MOD as the first three letters, we can arrange the remaining 3 letters in 3! ways = 6 ways.
Adding these to the total, we have 312 (previous total) + 6 (for MOA) + 6 (for MOD) = 324 words.
Next, we consider words that start with 'MON'. The word 'MONDAY' comes after 'MONAD' in dictionary order, so:
- Fix 'MONA' as the first four letters. The remaining 2 letters ('D' and 'Y') can be arranged in 2! ways, which gives us 2 more words: 'MONADY' and 'MONAYD'.
So, we have 324 (previous total) + 2 (for 'MONADY' and 'MONAYD') = 326 words.
Finally, we have the word 'MONDAY' itself, which is the 327th word.
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