3. In a school, all 300 Secondary 3 students study either Geography, Biology or both Geography and Biology. If $80 \%$ study Geography and $50 \%$ study Biology, how many students study both Geography and biology?
(A) 30;
(B) 60;
(C) 80;
(D) 90 ;
(E) 150 .
3. Ans: (D)
Use Inclusion and Exclusion Principle.