Voltage to current ratio in a circuit
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Calculation

First we find out how much of the current flows through one of the resistors, I choose the 10 Ohm one:

$$I_Vo = I_s * \frac{20}{10 + 20}$$

now we can find \$V_0\$ by using Ohm's law: 

$$V_0 = 10 * I_Vo$$

now we have both \$V_0\$ and \$I_s\$ and we can find the relationship:

$$\frac{V_0}{I_s} = \frac{10*I_Vo}{\frac{30*I_Vo}{20}} = \frac{20}{3}$$