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October 23rd, 2017, 04:12 PM  #1 
Newbie Joined: Oct 2017 From: India Posts: 3 Thanks: 0  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. Last edited by skipjack; October 24th, 2017 at 03:22 AM. 
October 24th, 2017, 03:34 AM  #2 
Global Moderator Joined: Dec 2006 Posts: 20,389 Thanks: 2015 
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  
Senior Member Joined: Sep 2015 From: USA Posts: 2,382 Thanks: 1281  Quote:
$A$ is the set of all numbers using digits $37$ but NOT $2$ $B$ is the set of all numbers using digits $26$ 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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