Terry is sitting 10 meters above sea level, and the fish are swimming 30 meters below Terry. To find the depth at which the fish are swimming, you need to combine the height of Terry above sea level and the depth of the sea below Terry.
When Terry is 10 meters above sea level and the fish are swimming 30 meters below Terry, you can think of it as Terry being at 0 meters, the sea level being at 10 meters, and the fish swimming 30 meters below sea level.
To calculate the depth at which the fish are swimming, you can add Terry's height above sea level (10 meters) to the depth below sea level where the fish are swimming (30 meters).
Therefore, the fish are swimming at a depth of 10 meters (Terry's height above sea level) + 30 meters (depth below sea level) = 40 meters below the sea level.
