R1(ABD) , key is A, No.of tuples = 50.
R2(BCE) , key is B, No.of tuples = 40.
R3(CF), key is C, No.of tuples = 80.
Maximum size of R1*R2*R3 = ?? , where * denotes natural join.
First lets compute R1*R2, where common attribute is B, which is a key of R2. So B represents here foreign key for table R1 which refers the primary key of table R2. So every element of attribute B in R1 must be there in R2. So Maximum tuples return by R1*R2= 50.
Similarly, R1*R2 ( ABCDE) and R3 ( CF) have common attribute C, which is a key of R3. So C represents foreign key for table R1*R2 which refers the primary key of table R2. So every element of attribute C in R1*R2 must be there in R3 and here R1*R2 can have maximum 50 tuples where elements of attribute C in R1*R2 match with the 80 elements of attribute C in R3. So Maximum tuples return by R1*R2*R3 = 50.