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)=1Pr( Not(junior or Psy).
Finally, then
Pr(Junior ANd Psy)=Pr(Jun)+Pr(Psy)Pr(Jun or Psy)
= Pr(Jun)+ Pr(Psy)(1Pr(NOT(Jun or Psy))
= 13/50 + 20/501+24/50
=57/501= 7/50
so 7 Psy majors are Juniors.
check my thinking.
This is not physics. Please don't misrepresent your subject again.
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