\(x_1=A \sin \left(\omega t+\phi_1\right)\)
\(x_2=A \sin \left(\omega t+\phi_2\right)\)
\(x_1-x_2=A \sin \left(\omega t+\phi_1\right)-A \sin \left(\omega t+\phi_2\right)\)
\(20=2 \times 20 \sin \left(\frac{\phi_1-\phi_2}{2}\right) \cdot \cos \left[\omega t+\left(\frac{\phi_1+\phi_2}{2}\right)\right]\)
\(\frac{1}{2}=\sin \left(\frac{\phi_1-\phi_2}{2}\right) \cdot \cos \left(\omega t+\left(\frac{\phi_1+\phi_2}{2}\right)\right) \text { for maximum value. } \Rightarrow \frac{\phi_1-\phi_2}{2}=\frac{\pi}{6} \Rightarrow \phi_1-\phi_2=\frac{\pi}{3}\)