what is Onto function with example ?

A function f:X→Y is said to be an onto or surjective function if rng f =f(X)=Y i.e if every element of Y is the image of some element of X.

e.g f:Z→Z,  y=f(x)=x+2 is an onto function

Because for every y belongs to Z set, x  belongs to Z set also so the given function is an onto function.