Clarinex_D is a medication whose purpose is to reduce the symptoms associated with a variety of allergies. In clinical trails of Clarinex_D , 5% of the patients in the study experienced insomnia as a side effect. A random sample of 20 Clarinext_D users is obtained , and the number of patients who experienced insomnia is recorded.

Please find the probability that at least 3 experienced insomnia as a side effect. (when you answer this question, please specify what is the n and what is the r )