There is a difference of 4 min on travelling each longitude. 4 min add on moving 1 degree east and subtract on moving 1 degree west. there is a difference of 111.11 km between 1 degree longitudes at the equator. so on multiplying 111.11 with 360, we get the diameter of the earth at the equator
we can get the difference of longitudes between each time zone by dividing 360 by 24 hours= 360/24= 15 degrees.