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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.
