Ok, so V=IR, and V/R = I. Ohms Law.
There will a current flowing thru both resistors.
V/(photoresistor + 10000) = Itotal
The voltage across each resistor is then
V1= Itotal * photoresistor,
Vout = Itotal * 10000,
Vin = V1 +Vout
Put it all together, do some algebra, and you get
Vout = Vin * R2/(R1 +R2)
(and Vin = V1 + Vout, or Vin - Vout = V1)
R1 is the resistor between Vin and Vout (photoresistor), and R2 is the resistor between Vout and Gnd (10000).