The resistors are connect in series, because in parallel the potential drop across all resistors will have been the same and will be equal to the battery voltage i.e. 60 V.
First we need to find the equivalent resistance and the current passing through the circuit.
Since, resistors are in series, equivalent resistance will be the sum of all individual resistances. So,
Equivalent Resistance = 20 + 40 + 60 = 120 ohm Current through the circuit = Voltage / Equivalent Resistance = 60/120 = 0.5 ampere
The voltage drop across R3 =Resistance of R3 x Current = 60 x 0.5 = 30 V
