Two resistors of 100 Ω each are connected in series with a 9 V battery. A voltmeter of 400 Ω internal resistance is connected to measure the voltage drop across one of the resistors. The voltmeter reading is ____ V.

A DC source supplies a voltage of 15 V to two series-connected resistors, one of which is 7.5 Ω and the other is an unknown resistor R. If the total circuit current is 1 A, determine the value of the unknown resistance R.

A resistor of 10 Ω is connected to a constant voltage source. When another resistor is connected in series with it, the current in the circuit becomes half of its initial value. Assuming the supply voltage remains unchanged, the resistance of the second resistor is _______.