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October 23rd, 2017, 04:12 PM   #1
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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?
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Last edited by skipjack; October 24th, 2017 at 03:22 AM.

 October 24th, 2017, 03:34 AM #2 Global Moderator   Joined: Dec 2006 Posts: 21,026 Thanks: 2257 No. Why would you think that?
 October 24th, 2017, 11:02 AM #3 Newbie   Joined: Oct 2017 From: India Posts: 3 Thanks: 0 Can you please explain why it would be so I am totally not able to understand.
October 24th, 2017, 12:13 PM   #4
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Quote:
 Originally Posted by darknoxx 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?

 October 25th, 2017, 01:26 AM #5 Newbie   Joined: Oct 2017 From: India Posts: 3 Thanks: 0 Thanks you so much for your answer.

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