What is the distance between 2 cities? Both Quito and Kampala lie on the Equator. The longitude of Quito is 82°30'W and that of Kampala is 37°30'E. What is the distance from Quito to Kampala?
A) along the shortest surface path
B) along a direct through the Earth path?
Please help me visualize it with a figure. How to do it?
This is hardly an astronomy question, but I like drawing, so here you are:



Longitudes are measured from the Greenwich meridian, so the angle between Kampala and Quito is
$$\theta_\mathrm{Q} + \theta_\mathrm{K} = 82.5^\circ + 37.5^\circ = 110^\circ.$$
(remember that $0^\circ30' = 0.5^\circ$).
The shortest surface path is along Equator. Since $110^\circ$ is $\frac{110^\circ}{360^\circ} \simeq 0.3$ times the circumference of Earth at Equator, the length of path A (the dashed line) is
$$\mathrm{A}:\,\,d = 0.3 \times 40,075\,\mathrm{km} = 12,245\,\mathrm{km}$$

For path B (the solid line), you need a bit more  trigonometry. The radius of Earth is $R = 40,075\,\mathrm{km}\,/\,2\pi = 6,378\,\mathrm{km}$. The rest will be left as an exercise.