Answer:
11
Step-by-step explanation:
You want to know the number of students with no brother or sister, given that 12 of 28 have at least one brother, 13 have at least one sister, and 8 have both.
Two-way table
The attachment shows the two-way table that can be formed from the given information. The four given numbers are shown in green at lower right. The other numbers in the table are filled in to make the totals accurate.
The number of students not having either a brother or sister is 11.
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Additional comment
In terms of probability, which is the fraction that each event is of the total, you have ...
P(B∪S) = P(B) +P(S) -P(B∩S) . . . . . . . useful relation to memorize
P(B'∩S') = 1 -P(B∪S) = 1 -(12/28 +13/28 -8/28) = 11/28
This tells you 11 of the 28 students do not have a brother or sister.
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