Ben and Cam are scuba diving. Ben is 15.8 meters below the surface of the water. Cam is 4.2 meters above Ben. What is Cam's
position relative to the surface of the water?

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Explanation:

Check out the diagram below.

Draw a vertical number line with 0 at the center. The positive values are above it, while the negative values are below it.

Between -15 and -16, closer to -16, plot the value -15.8 to indicate Ben's position. I have done so as the point B.

We move 4.2 units up to arrive at Cam's position

-15.8 + 4.2 = -11.6

So Cam is 11.6 meters below the surface of the water.