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).