Karnaugh Maps

[dinumadushan]

 

[jamiezue]

ohhh crap thats not it.. getting the right answer aint enough... :S they want me to do a presentation explaining how i got the answer...
i know how to draw the karnaugh map from the truth table, deriving the boolean expression and circuit and all the rest of it.... but what i dont get is the Truth table... how did you guys get that Y output? how do we know if its true or not?

it's not our job to decide the nature of Y output (true or false) . the question always gives us the value..(nature) of each instance. In above question they tell us whenever prime number is presented the Y output would be true..