Two resistors of 10 and 20 are connected in series and this combination is connected across a 30 V supply voltage.
Find the voltage across the 10 resistor.

Correct option is A

When resistors are in series, the same current flows through each resistor. The voltage drop across each resistor is proportional to its resistance based on Ohm’s Law (V = IR).
The total resistance in the circuit is 10 Ω + 20 Ω = 30 Ω. The supply voltage is 30 V, so the total current I in the circuit is
$$I = \frac{\text{Total Voltage}}{\text{Total Resistance}} = \frac{30}{30} = 1$$​
The voltage drop across the 10 Ω resistor is thus
$$V_{10\Omega} = I \times 10 \, \Omega = 1 \times 10 = 10 \text{ V}.$$​