The key to solving these problems was to read each one carefully to determine the real question so that you could properly set up the equation. The identity of the letter writer is the key to the style and content of your response. All letters deserve a thoughtful response, but a letter from a senior scientist will elicit a response that will differ from your response to a letter from a child in elementary school.
Many letter writers ask one or more specific questions.
Carefully list or underline each question so that you can be sure to cover each one in your response. Other correspondents want general information. Giving such letters a somewhat general response is all right. We want to be responsive to individuals, but we are not required to do an individual's research. However, we can refer them to some of the material cited on page 25.