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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 -

    Draw your Venn Diagram

    My drawing shows that

    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 don't misrepresent your subject again.

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