What is the change in elevation from 140 feet above sea level to 40 feet below sea level? Explain

140 feet above sea level can be seen as + 140 because it is 140 higher than sea level which is 0.

40 feet below sea level can be seen as - 40 because it is 40 below 0.

The difference would be 180.

This is best seen when visualizing a numberline. You start at the 140 mark and go all the way to negative 40 resulting in 180.