Give an example of a function from A to B that is not one-to-one. Explain why it is not one-to-one A= {a,b,c,d} B= {1,2,3,4,5}
Currently studying for a final. I know that a one-to-one function cannot map to 2 elements. There are more elements in B than in A. I don't know how to give a specific example of a function that is not one-to-one, though. How do I know which element maps where? 
Seems very easy. Any help is appreciated.
 A: The most obvious non-one-to-one functions are the constant functions, which take the same value for every input. One constant function $f:A\to B$ is the one given by
$$
f(a) = f(b) = f(c) = f(d) = 2
$$
It is not one-to-one because while $a \neq b$, we have $f(a) = f(b)$.
A: For a one-to-one function we have
$f(x_1) = f(x_2) \Rightarrow x_1 = x_2$
Hence this is not one-to-one
$f: A \to B$
$a \mapsto 1$
$b \mapsto 1$
$c \mapsto 2$
$d \mapsto 3$
because $f(a) = f(b)$ but $a \not = b$

"How do I know which element maps where?"
You are supposed to give an example of a function so its
up to you to decide which elements map to which elements.
A: You can define a mapping element-wise. That is,
Let $f : A \to B$ be defined as follows 
\begin{align}
a &\mapsto 1\\
b &\mapsto 1\\
c &\mapsto 1\\
d &\mapsto 2.\\
\end{align}
Then $f$ is not one-to-one. 
Of course, you can do this for whatever elements you like.. as long as there is at least one element such that $f(a) = 1$ and $f(b) = 1$, then it is not one-to-one.
A: Based on your statement here:

There are more elements in B than in A.

What I think may be confusing you (and I may be wrong, of course) and what no one else has said yet is that your function doesn't need to hit every element of $B$.
And it is true that in order to be a function, one element in $A$ cannot go to more than one element of $B$.
However, it is allowed for more than one element of $A$ to go to one element of $B$.  This does not violate the definition of being a function.  In fact, it is precisely this scenario that gives you a function that is not one-to-one (because many elements in $A$ go to one element in $B$).
Regarding your question:

How do I know which element maps where?

It is entirely up to you, as long as your function actually is a function (i.e., it doesn't send one element of $A$ to more than one element of $B$).  Here's a simple example, much like Arthur's:  Define a function $f$ so that every element of $A$ gets sent to the element $1$ in $B$.  Then, specifically, you have:
$$
  f(a) = 1, \qquad f(b) = 1, \qquad f(c) = 1, \qquad f(d) = 1
$$
This is a perfectly valid function.  And it is not one-to-one because different elements in $A$ go to the same element in $B$.
A: Elaboration on "There are more elements in B than in A." this statement does not concern itself with wither a function from A to B is 1-1. 
Consider A = {a} and B = {1,2,3,4,5}. Here every function from A to B is one to one. Possible functions are F(a)=1,G(a)=2,H(a)=3,I(a)=4,J(a)=5 which are all one to one.
Consider A = {a,b,c,d} and B = {1,2,3,4,5}. F(a) = 1 F(b) = 1 F(d) = 1 F(c) = 3. This is not a one to one function which is also an example to your question
"There are more elements in B than in A." Implies that a function from A to B can't be surjective or onto because B will never be filled up. This can be tricky when dealing with infinite sets however.
