Discrete Math Structure -- Set Operation. I asked in a previous question what is the "c" in c(A), and @tamtoan answered that it is for count. I was able to figure out the first question, but can't get the correct answer for the second question. The second question is: If c(A Union B) = 17, c(A Intersect B) = 3, and c(A) = c(B), then c(A) = ____? So, intersection is 3. Union is 17, which means, c(A) and c(B) should have 7 each right? I entered 7, but the result shows it up as incorrect. Did I make a silly mistake somewhere? _

Question

Discrete Math Structure --> Set Operation. 

I asked in a previous question what is the "c" in c(A), and @tamtoan answered that it is for count. I was able to figure out the first question, but can't get the correct answer for the second question. 

The second question is:
If c(A Union B) = 17, c(A Intersect B) = 3, and c(A) = c(B), then c(A) = ____? 

So, intersection is 3. Union is 17, which means, c(A) and c(B) should have 7 each right? I entered 7, but the result shows it up as incorrect. Did I make a silly mistake somewhere? >_< Thank you.

tamtoan · Answer

it's not 7 each :)...if 7 each in both A and B..you will get at most 14 for the union :)...remember union is 17 and that is less than count of A + count of B ...try drawing some pair of circles ..with intersection 3 first...and distribute other elements so that you can have a union of 17...i think you can figure it out :) if not, can check with me again..i think i got this

ajprincess · Answer

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May be u can this formula