Solution

Let monthly income of MR. X = $Rs. 100x$

From I & II : Monthly SAVINGS = 100 - 85 = 15 %

=> $\frac{15}{100} \times 100x = 4,500$

=> $x = \frac{4500}{15} = 300$

$\therefore$ Monthly salary = $100 \times 300 = Rs. 30,000$

From I & III : TOTAL money spent

=> $\frac{85}{100} \times 100x \times \frac{4}{5} = 20,400$

=> $x = \frac{20,400}{68} = 300$

$\therefore$ Monthly Salary = $100 \times 300 = Rs. 30,000$

Thus, statement I is necessary and either II or III is not necessarily REQUIRED.