Answer:
A, 30V
Explanation:
First combine all resistors to an equivalent resistor. Since they are in series, the equivalent resistance is the sum of all resistor
Req = 20 + 40 + 60 = 120Ω
Using Ohm's law, find the current in the circuit
V = I * R
I = V / R
I = 60V / 120Ω
I = 0.5 A
Now the potential drop across the resistor R3 is the current times R3 resistance, therefore:
Vdrop = 0.5A * 60Ω = 30V
So the potential drop across resistor R3 is 30 V
