The following questions are accompanied by three statements (I), (II) and (III). You have to determine which statement(s) is/are sufficient/necessary to answer the questions.

Four friends P, Q, R and S are employee of a software company. What is the difference between P and R’s income? 

I. Sum of monthly income of P, Q, R and S is Rs. 25000. 

II. 20% of Q’s income is equal to difference between P and R’s income and monthly income of Q is 6000.

III. Monthly income of S is 8000.