Voltage divider not making any sense
Alright let's get the Math correct and then we can decide the Resistor values. The circuit above is a simple voltage divider circuit. Basically, the two resistors are connected in series and we try to measure (or read) the voltage from one of the resistors (here the one whose other end is connected to Reference (or lets say Ground). You can visualize the circuit as a simple series circuit the one shown below - 







simulate this circuit &ndash; Schematic created using CircuitLab

We want to measure the voltage across R2 resistor. Since both the resistors are connected in series the current flowing through them will remain same. Therefore the voltage across R2 comes out to be - 

V0 = R2* I  ..... (1)

But we don't know the value of I. Well getting that is easy enough. The net resistance in series is the sum of the resistance connected in series. Therefore Rnet = R1 + R2. 
As a result the current the circuit comes out to be- 

I = V1/Rnet ..... (2)

Using this value in (1)

V0 = R2 * V1/(Rnet) 

=> V0 = R2 * V1/ (R1+R2) 

..................................................................................

In your case 
V1 = 10, 12 or 15 (you have a dynamic supply) 
and you want V0 to be around 5V. 

Let's take R1 to be 10k ohm. Using the result above
5 = R2 * 15/(10k + R2)

Solving the above equation gives R2 as
5k ohm

So, you can use R1 as 10k and R2 as 5k (4.7k as 5k is not a standard value) and your input should be 5V. This should give you a voltage of about 5V across R2. 
I hope it helps.