Bunuel
If N denotes the number of different selections of 5 letters from the word MISSISSIPPI then N belongs to the set
(A) {15, 16, 17, 18, 19}
(B) {20, 21, 22, 23, 24}
(C) {25, 26, 27, 28, 29}
(D) {30, 31, 32, 33, 34}
(E) {35, 36, 37, 38, 39}
Solution:
In MISSISSIPPI, we see that there are 1 M, 2 P’s, 4 I’s and 4 S’s. If 5 letters are chosen, they can consist of 2, 3, or 4 distinct letters. Let’s look at the each case:
Case 1: 2 distinct letters
MI, MS, (PI), (PS), [IS]
Case 2: 3 distinct letters
(MPI), (MPS), {MIS}, <PIS>
Case 3: 4 distinct letters
{MPIS}
In the above cases, letters without anything means there is only 1 selection.
Letters within parenthesis means there are 2 selections. For example, (PS) could be 1 P and 4 S’s OR 2 P’s and 3 S’s.
Letters within braces means there are 3 selections, For example, {MIS} could be 1 M, 1 I and 3 S’s OR 1 M, 2 I’s and 2 S’s OR 1 M, 3 I’s and 1 S.
Letters within brackets means there are 4 selections. For example, [IS] could be 1 I and 4 S’s OR 2 I’s and 3 S’s OR 3 I’s and 2 S’s OR 4 I’s and 1 S.
Lastly, letters within angle brackets means there are 5 selections. For example, <PIS> could be 1 P, 1 I and 3 S’s OR 1 P, 2 I’s and 2 S’s OR 1 P, 3 I’s and 1 S OR 2 P’s, 1 I and 2 S’s OR 2 P’s, 2 I’s and 1 S.
Therefore, case 1 has 1 + 1 + 2 + 2 + 4 = 10 selections; case 2 has 2 + 2 + 3 + 5 = 12 selections and case 3 has 3 selections for a total of 10 + 12 + 3 = 25 selections.
Answer: C