Why is the last line of longitude 75 degrees?

Answer 1

There is no "last line" of longitude.

The longitude of points on the earth's surface ranges from zero to 180 degrees east

and zero to 180 degrees west. There is no number within this range that doesn't

describe billions of points, and no place on earth that doesn't have a longitude.

Your map or globe may include a few widely spaced lines to indicate selected longitudes,

or it may not. If it does, you should not fall into the trap of believing that those are the

only longitudes that exist.

Answer 2

The last line of longitude is set at 180 degrees, not 75 degrees. This is because a full circle is divided into 360 degrees of longitude (half a circle is 180 degrees). So, the last line is at 180 degrees longitude, marking the endpoint of the longitudinal coordinates around the Earth.