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# In how many of the distinct permutations of the letters in MISSISSIPPI do the four I’s not come together?

Question:

In how many of the distinct permutations of the letters in MISSISSIPPI do the four I’s not come together?

Solution:

In the given word MISSISSIPPI, I appears 4 times, S appears 4 times, P appears 2 times, and M appears just once.

Therefore, number of distinct permutations of the letters in the given word

$=\frac{11 !}{4 ! 4 ! 2 !}$

$=\frac{11 \times 10 \times 9 \times 8 \times 7 \times 6 \times 5 \times 4 !}{4 ! \times 4 \times 3 \times 2 \times 1 \times 2 \times 1}$

$=\frac{11 \times 10 \times 9 \times 8 \times 7 \times 6 \times 5}{4 \times 3 \times 2 \times 1 \times 2 \times 1}$

$=34650$

There are 4 Is in the given word. When they occur together, they are treated as a single object for the time being. This single object together with the remaining 7 objects will account for 8 objects.

These 8 objects in which there are $4 \mathrm{Ss}$ and 2 Ps can be arranged in $\frac{8 !}{4 ! 2 !}$ ways i.e., 840 ways.

Number of arrangements where all Is occur together = 840

Thus, number of distinct permutations of the letters in MISSISSIPPI in which four Is do not come together = 34650 – 840 = 33810