(i) Number of 4-letter words that can be formed from the letters of the word MONDAY, without repetition of letters, is the number of permutations of 6 different objects taken 4 at a time, which is 6P4​.Thus, required number of words that can be formed using 4 letters at a time is6P4​=(6−4)!6!​=2!6!​=2!6×5×4×3×2!​=6×5×4×3=360 (ii) Number of words that can be formed by using all the letters of the word MONDAY at a time is the number of permutations of 6 different objects taken 6 at a time, which is 6P6​=6!.Thus, required number of words that can be formed when all letters are used at a time is 6!=6×5×4×3×2×1=720.(iii) In the given word, there are 2 different vowels, which have to occupy the rightmost place of the words formed. This can be done only in 2 ways.Since the letters cannot be repeated and the rightmost place is already occupied with a letter (which is a vowel), the remaining five places are to be filled by the remaining 5 letters. This can be done in 5 ! WaysThus, in this case, required number of words that can be formed is5!×2=120×2=240