Runway numbers are determined by rounding the compass bearing of one runway end to the nearest 10 degrees and truncating
the last digit, meaning runways are numbered from 1 to 36. The opposite end of the runway
always differs by 180 degrees (because its a straight line 😉) and will therefore always differ by 18.
Use Google maps and search for the number at the end of any airport in the world and amaze your friends by telling them the value at the other end.