Final answer:
Casey dove a total of 18.42 feet below sea level by adding her initial dive of 11.75 feet to her second dive of 6 2/3 feet.
Step-by-step explanation:
To determine how far below sea level Casey dove, we need to add the two depths she reached during her dive. Initially, she dove 11.75 feet. After taking a break, she then dove an additional 6 2/3 feet, which is equivalent to 6.67 feet when converted to a decimal. To find the total depth, we add these two values together:
11.75 feet + 6.67 feet = 18.42 feet
Therefore, Casey dove 18.42 feet below sea level.