In an institute employing managers, observers and assistants, what is the monthly salary of an assistant ?
I: Each observer gets Rs. 12000 per month more than an assistant.
II: An observer and an assistant together get Rs. 32000 per month.
III: The total salary per month of a manager and an observer is Rs. 57000.

Clearly, in each statement we cannot find the monthly salary of an assistant.

I & II : Let salary of an assistant = x

=> salary of an observer = 12000 + x

=> 12000 + x + x = 32000 [From II]

=> x = 10,000.

Thus, I & II are sufficient.

I & III : Let salary of an assistant = x and manager = y

=> salary of an observer = 12000 + x

=> 12000 + x + y = 57000 [From III]

Since, there are two variables and one equation, we cannot find the required answer.

Thus, I & III are not sufficient together.

Similarly, II & III are not sufficient together.