Ascuba diver dives to a depth of 50 feet, sinks 12 feet, then rises 20 feet. what is the diver's final position relative to sea level?

We can use a number line to help us solve, but I guess "visualizing" it with a vertical number line works too. 

We start at the point 0. We go down to the point -50, after the diver dives down to a depth of 50 feet. "Sink" is to go down, so we go down to -62 because -50-12=-62. "Rise" is to go up, so we go up to -42 because -62+20=-42.

The diver's final position relative to sea level would be at 42 feet.