In this problem, I'm assuming that there zero students studying neither Algebra nor Biology.
First, get the number of students(x) who are studying both Algebra and Biology:
16+21-x = 30
37-x = 30
x= 7
Then we have this Venn Diagram:
The famous formula to solve P(A or B):
P(A or B) = P(A) +P(B) -P(A and B)
P(A) = 16/30
P(B) =21/30
P(A and B) = 7/30
P(A or B) = 16/30 + 21/30 - 7/30 = 30/30 = 1
- I hope you wrote the problem correctly and didn't miss anything.