One possible version:
Dear Angela,
How nice to hear from you! In your letter you asked me how to keep healthy. I would like to give you some advice.
As you know, we teenagers grow very fast, so we need plenty of nutritious foods, such as fish, meat, and beans, especially fresh vegetables and fruits. You’d better not eat too much food rich in sugar or fat. Besides, you should take physical exercise regularly instead of studying in the classroom all the time. For example, you can play basketball with your friends or swim after school. What’s more, having enough sleep is very important. So I advise that you should have at least eight hours of sleep every day and not stay up too late in the evening. Only in this way can you keep fit.
Yours
Angela
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