Question

Two places lies on the same latitude, and their longitudes are 10°W and 30°W . Find the distance between them

Answer 1

depends on how you travel. If you are going to stay at the same latitude, then
earth's radius is about 6400 km
at latitude of θ degrees, the radius is 6400 cosθ
so the distance would be 6400 cosθ * 20/360 km

but the shortest distance would be along the great circle joining the two points. That's a bit trickier to calculate. Read up on spherical trig.

Answer 2

Not enough information.
the difference in the given longitudes is 20°
At the equator 1° of longitude is appr 111, but since these line converge
as the latitude increases, we can't find the answer.

If given two sets of coordinates, the distance between them is not a simple
solution, depending on the curvature of the earth. That is why a plane flying
from Frankfurt to Los Angeles would fly over northern Canada, due to that
shortest distance.

Answer 3

I think might multiply oobleck's solution by 2 pi, and yes the great circle route is shorter but not by much.

Answer 4

oops. Thanks, @Anonymous