CONCEPT: It’s a word problem from Arithmetic.

SOLUTION:
 

The number of members in A on Sept 2, 2007:
=n+4m=n+80n=81n (As,on the first day of each subsequent month, m members join A and m=20n)
Number of members in B on Sept 2, 2007:
=n× p× p× p× p (As,on the first day of each subsequent month, number of members in B gets multiplied by p)
As given in the question, both organisations A and B have the same number of members on Sept 2,2007
81n=n×p4
p=3 (B)