Posted by **Kassie** on Monday, February 1, 2010 at 8:08am.

A class has 50 students. 13 are juniors, 20 are psych majors, and 24 are neither psych majors nor juniors. How many psych majors are juniors?

- Probability -
**bobpursley**, Monday, February 1, 2010 at 8:50am
Draw your Venn Diagram

My drawing shows that

Note:

Pr(Junior or Psy)= Pr(Jun)+ Pr(Psy)-Pr(Junior AND Psy)

But you are given Pr (NOT(Junior Or Psy), so Pr(Junior or Psy)=1-Pr( Not(junior or Psy).

Finally, then

Pr(Junior ANd Psy)=Pr(Jun)+Pr(Psy)-Pr(Jun or Psy)

= Pr(Jun)+ Pr(Psy)-(1-Pr(NOT(Jun or Psy))

= 13/50 + 20/50-1+24/50

=57/50-1= 7/50

so 7 Psy majors are Juniors.

check my thinking.

This is not physics. Please dont misrepresent your subject again.

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