A mixed circuit is formed by components (voltage sources and / or current, resistors, inductors or capacitors) connected in series and parallel. Consider a simple example:
In this example we can not add R2 to R1 because R1 is being affected by R3 and R1 is in parallel with R3. We will reduce the circuit, taking an equivalent resistance between R1 and R3
Using Rt we can obtain the total current: It=V/Rt=12V/18Ohm=0,66A
Now we have obtained the current value for all the circuit. For obtaining the individual value of the currents and voltages of each element, we apply the same method:
We divide this voltage between resistors R1 and R3: I=3,96V/12Ohm=0,33A ; Since R1 and R3 are equal, the current will be the same in R3.
Conclusions: Of the total current(0,66A) it can be observed that it is divided between R1 and R3(0,33A), and we also can see that the current is distributed between the circuit elements.