The total number of longitudes is 180+180+1(0° Prime Meridian)-1(180E and 180W are same longitudes) = 360. There are total 180 west longitudes and 180 east longitudes. The 180E and 180W longitude is the same longitude and hence, we have subtracted 1 from 360. The 0° longitude, which is called the Prime Meridian is added to the total count making it 360.