A 12.0 V battery is connected to two resistors in series:

R1 = 12.00 , R2 = 4.00 .

The Req is 16 ohm, total current is same for both i. e. 0.75A and voltage are 9V and 3V for resistor R1 and R2 respectively.

Explanation:

For series combination, resistance are added, Req = R1 + R2 = 16 ohm.

V (total) = I (total) * Req

or, I (total) = V/Req=12/16=0.75 amp.

Now,

Voltage across R1 = I (total) * R1 = 0.75 * 12=9V

Voltage across R2 = I (total) * R2 = 0.75 * 4=3V

However, in parallel combination, voltage is constant across both the resistance, current is different in both the resistance and total resistance is calculated by the following expression: R (parallel) = (R1 * R2) / R1+R2