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?

Answer 1

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 don't misrepresent your subject again.