Final answer:
The text does not provide details on how Kate knew the depth of the crack; we can only guess that she might have estimated it or measured it in some unmentioned way. The passage focuses more on her experiences and her environment.
Step-by-step explanation:
The context provided does not contain enough information to definitively explain how Kate knew the crack was 10 and a half inches deep. However, based on the text, it seems that the focus is on Kate's observations and experiences in her cell rather than on any specific measurement of a crack's depth. Without additional information, such as Kate using a tool or a method to measure the crack, we can only speculate that she might have estimated the depth based on her knowledge or other measurements she could have taken inside her cell. The passage does give us insight into Kate's environment and her attention to detail, but it does not describe the action of measuring a crack's depth.