Calculate the reading on V1.

This is a series circuit.

In a series circuit, the total resistance is the sum of individual resistances.
$R_{total} = R_1 + R_2$
$R_{total} = 10 \, \Omega + 20 \, \Omega$
$R_{total} = 30 \, \Omega$

$I_{total} = \frac{V_{total}}{R_{total}}$
$I_{total} = \frac{12 \, V}{30 \, \Omega}$
$I_{total} = 0.4 \, A$

V1 measures the voltage drop across $R_1$.
The voltage drop across $R_1$ ($V_1$) is calculated using Ohm's Law:
$V_1 = I_{total} \times R_1$
$V_1 = 0.4 \, A \times 10 \, \Omega$
$V_1 = 4 \, V$

The reading on V1 is 4 V.