Question

P and Q are two points on latitude 55 degrees North and their longitudes are 33 degrees West and 23 degrees East respectively. calculate the distance between P and Q measured along (a)along the parallel of latitude (b)along a great circle

Answer 1

Let R be the radius of the earth.
The radius of a parallel at latitude θ is r = R cosθ
arc length s = rØ where Ø is the difference in longitude.

getting the great circle distance is a bit trickier, but as usual, google is your friend. Here's a calculator, and also the formulas used to do the math.

https://keisan.casio.com/exec/system/1224587128