Question: How do i write a letter of interest? 

Answer: Depends on the subject you are applying to. See what the subject is about e.g. if you are applying for a degree course in art history, then read up a bit on it, write a short introduction as to what you have done sofar related to studies. Then write about in the contents paragraph about why you want to study this particular subject, at particular college etc. and wind it up by telling them how good you will be in it and how you and college will benefit from your getting admission there.
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