F.5 physics electric circuits

Since the voltmeter has very high resistance, current flowing through it is negligible ( = zero).
Since reading is 0.4V, so current flowing through the 2 ohms resistor
= 0.4/2 = 0.2A.
The same current flow through all resistors,
so (2 + 3 + R)(0.2) = 1.5V
1 + 0.2R = 1.5
0.2R = 0.5
R = 2.5 ohms.
(b)
When switch K is closed, the 3 ohm resistor is shorted. So resistance of the circuit becomes 2 + R = 4.5 ohms.
So current becomes 1.5V/4.5 ohms = 1/3A
So reading of voltmeter = voltage across the 2 ohm resistor = (1/3)(2) = 2/3V.