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The formula is Total = A+B+C - sum of exactly two + 2*all three + neither
21+18+19-x-2*5=39
solving for x you get 9
Answer D
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Could someone explain to me why it is necessary to double the amount of parents in all groups?

Thanks
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A+B+C - sum of exactly two - 2*all three
ans: D
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Formula for three sets is Total = n(A) + n(B) + n(C) – n(A and B) – n(B and C) – n(C and A) + n(A and B and C)
Which implies 39 = 21+18+19 – n(A and B) – n(B and C) – n(C and A) + 5
Therefore n(A and B) + n(B and C) + n(C and A) = 24
Between A & B there are 5 parents participating in C & similarly the same thing is happening between A & C and B & C.
Therefore correct answer is 24-15=9
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Let exactly one= A
Let exactly two= B
Let exactly three=C

So, A+B+C=39-----------I
A+2B+3C=58----------II

from I and II
B+2C=19
B=19-10 [C is given 5]
B=9
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