Solution:

Dynamic resistance is defined as \( r_d = \frac{\Delta V}{\Delta I} \). Here, \( \Delta V = 1.2 \text{ V} - 1 \text{ V} = 0.2 \text{ V} \) and \( \Delta I = 20 \text{ mA} - 10 \text{ mA} = 10 \times 10^{-3} \text{ A} \). Thus, \( r_d = \frac{0.2}{10^{-2}} = 20 \ \Omega \).