EXAMPLE PROBLEM ON RESISTORS IN COMBINATION CIRCUITS
Question
Figure 1 Combination Circuit 1 
Hints

Solution
This circuit is composed of 3 'elements' connected in series: the group
of parallel resistors between A and B, the single resistor R3,
and the group of parallel resistors between C and D.
First, we will find the equivalent resistance between A and B.
Here, we have two resistors, R1 and R2,
connected in parallel. Using the formula for resistors connected in parallel:
Replacing the two parallel sections with their equivalent
resistances, and redrawing the circuit, we get the circuit in Figure 2.
We see that there are three resistances connected in series: RAB,
R3, and RCD. Using the formula
for resistors in series,

Figure 2 Simplified Version of Circuit 1 