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In a class 30 of students, 22 have a brother and 25 have a sister. There are 2 students who do not have any siblings. What is the probability that a student chosen randomly from the class does not have a sister?

 Mar 15, 2020
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You should do this with a Venn diagram.

 

Two overlapping circles inside a rectangle

There are 2 with no siblings that leaves 28 with a brother or a sister or both.

Let x be the number with both.

22 have a brother so  22-x have a brother but no sister.

25 have a sister so 25-x have a sister but no brother

 

so we have    22-x  +  x  +  25-x = 28

 

Solve that, find x

Draw the vent diagram properly.

Find the desired probability.

 Mar 15, 2020

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