1500 is the correct answer.
Consider the following case:
I am representing the database table with relation having ordered pairs.
$R1(A,B)=\{(1,a),(2,a),(3,a)……..(49,a),(50,a)\}$
$R2(B,C)=\{(a,51),(a,52),(a,53)…….(a,78),(a,79),(a,80)\}$
Here the natural join of R1 and R2 will have 1500 rows.
This is because every row of R1 is paired with all the 30 rows of R2.