The nautical mile is defined so that one minute of latitude is one mile. A difference in latitude, in minutes, is a distance in miles. Move the two boats and watch the distance follow the angle.

Drag either boat along the meridian.

Boat A: N 18°15.0'
Boat B: N 46°05.0'
Difference in latitude: 27°50.0' = 1670′
Distance apart: 1,670 nm

Same side of the equator: subtract the smaller latitude from the larger.

From N 32°15.0′ to N 33°05.0′ is 50′ of latitude, so the boats are 50 nm apart.

From N 2°10.0′ to S 1°40.0′ crosses the equator: add them. 3°50.0′ = 230′ = 230 nm.

A minute of latitude is a nautical mile