Answer
$2$ km.
Work Step by Step
$38-18=10$ degrees per km. The difference in temperature is $20$ degrees. It is $2\cdot10$ degrees, thus the depth is $2$ km.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.