At time $t=10 \mathrm{s}$, the velocity of a particle moving in the $x-y$ plane is $\mathbf{v}=0.1 \mathbf{i}+2 \mathbf{j} \mathrm{m} / \mathrm{s} .$ By time $t=10.1 \mathrm{s},$ its
velocity has become $-0.1 \mathbf{i}+1.8 \mathbf{j} \mathrm{m} / \mathrm{s}$. Determine the
interval and the angle $\theta$ made by the average acceleration with the positive $x$ -axis.