MATH.
posted by UNKNOWN .
TWO series capacitors ( one 1 microFarad, the other of unknown value ) are charged from a 12 Voltage source. The 1 microFarad capacitor is charged to 8 Voltage and the other to 4 Voltage. What is the value of the unknown capacitor?

I don't see why they both don't charge to the full 12 V, if they are charged separately or in parallel.
What you seem to be talking about is charging them together in Series. In that case, the sum of the voltages is the supply voltage (12 V), and they both hold the same charge.
Therefore Q = C1*V1 = C2*V2
1 uF * 8 V = x * 4 V
x is the unknown capactitance, and you can solve for it.
x = (8/4) * 1 uF = 2 uF = 2*10^6 F
(uF stands for microFarads) 
Thankyou drwls.