the ways to arrange the letters of the word MISSISSIPPI is equal to

But what if we are asked to select and arrange, say, 5 of the letters from the word MISSISSIPPI?

Also, what if we are asked to find way to choose, say, 5 letters from the word MISSISSIPI? Is there a formula for indistinguishable items for combinations as well?

No formula ,at-least to my knowledge! Probably you’ll have to break it down to smaller cases and then solve those. For eg: In-case of the word “MISSISSIPPI” we can make the following cases to choose 5 letters :

all 5 selected letters are different;

one letter appears twice and three other letters appear once;

one letter appears three times and two other letters appear once; and

two different letters each appear twice.

Then we can use combinations to calculate the number of ways each case can happen.