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October 23rd, 2017, 05:12 PM   #1
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Permutation and Combination

Hey, guys I am new to this forum and I don't know whether this is the right way to ask a question or not.
I am able to understand the sum up until the n(A U B) part; shouldn't it be the total number of numbers which contain both the digits 2 and 7?
Thanks in advance.
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Last edited by skipjack; October 24th, 2017 at 04:22 AM.
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October 24th, 2017, 04:34 AM   #2
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No. Why would you think that?
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October 24th, 2017, 12:02 PM   #3
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Can you please explain why it would be so I am totally not able to understand.
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October 24th, 2017, 01:13 PM   #4
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Quote:
Originally Posted by darknoxx View Post
Hey, guys I am new to this forum and I don't know whether this is the right way to ask a question or not.
I am able to understand the sum up until the n(A U B) part; shouldn't it be the total number of numbers which contain both the digits 2 and 7?
Thanks in advance.
Here's what's going on.

$A$ is the set of all numbers using digits $3-7$ but NOT $2$

$B$ is the set of all numbers using digits $2-6$ but NOT $7$

so none of these numbers contain both $2$ AND $7$

thus the set of all numbers that don't contain both $2$ AND $7 = A \cup B$

can you follow the rest from there?
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October 25th, 2017, 02:26 AM   #5
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Thanks you so much for your answer.
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