Question

For 2 sets X={1,2,3,4},Y={−15,0,30},
f is a surjective function from X to Y. If f(1)=−15 and f(2)=f(4)=0,
what is f(3) ?

Answer 1

argggh,, must admit this is a new one for me

http://www.mathsisfun.com/sets/injective-surjective-bijective.html

Answer 2

every Y must be in f(X).
Since -15 and 0 are taken, we still need something to map to 30.

So, f(3) = 30.