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There are 5 letters and 5 directed envelopes. Find 'the number of ways in which the tetters can be pul into the envelopes so that all are not put in directed envelopes?
  • a)
    129
  • b)
    119
  • c)
    109
  • d)
    139
Correct answer is option 'B'. Can you explain this answer?
Most Upvoted Answer
There are 5 letters and 5 directed envelopes. Find 'the number of way...
To solve this problem, we can use the principle of inclusion-exclusion.

Let's consider the number of ways in which the letters can be put into the envelopes without any restrictions. In this case, each letter has 5 options, so there are a total of 5^5 = 3125 possible arrangements.

Now, let's calculate the number of ways in which at least one letter is put in its directed envelope. We can break this down into different cases:

1. One letter is put in its directed envelope: There are 5 options for the first letter, and once it is placed in its directed envelope, there are 4 options for the remaining letters. So, there are 5 * 4^4 = 1280 arrangements in this case.

2. Two letters are put in their directed envelopes: There are 5 options for choosing the two letters, and once they are placed in their directed envelopes, there are 3 options for the remaining letters. So, there are 5C2 * 3^3 = 90 arrangements in this case.

3. Three letters are put in their directed envelopes: There are 5 options for choosing the three letters, and once they are placed in their directed envelopes, there are 2 options for the remaining letters. So, there are 5C3 * 2^2 = 40 arrangements in this case.

4. Four letters are put in their directed envelopes: There are 5 options for choosing the four letters, and once they are placed in their directed envelopes, there is 1 option for the remaining letter. So, there are 5C4 * 1^1 = 5 arrangements in this case.

5. All five letters are put in their directed envelopes: There is only 1 way to arrange all the letters in this case.

Now, we can apply the principle of inclusion-exclusion. The number of ways in which at least one letter is put in its directed envelope is given by:

1280 - 90 + 40 - 5 + 1 = 1226

Finally, to find the number of ways in which all the letters are not put in directed envelopes, we subtract this value from the total number of arrangements:

3125 - 1226 = 1899

Therefore, the correct answer is option B) 119.
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Community Answer
There are 5 letters and 5 directed envelopes. Find 'the number of way...
  • Here, the first letter can be put in any one of the 5 envelopes in 5 ways. Second letter can be put in any one of the 4 remaining envelopes In 4 ways. Continuing in this way, we get the total number of ways in which 5 letters can be put into 5 envelopes
    = 5 x 4 x 3 x 2 x 1 = 120.
  • Since out of the 120 ways, there is only one way for putting each letter in the correct envelope.
    Hence, the number of ways of putting letters all are not in directed envelopes = 120 - 1 = 119 ways.
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There are 5 letters and 5 directed envelopes. Find 'the number of ways in which the tetters can be pul into the envelopes so that all are not put in directed envelopes?a)129b)119c)109d)139Correct answer is option 'B'. Can you explain this answer?
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