The Brainliest Answer!
topic for formal letters are 
write a letter to a book seller to ordering books
an application for a situation
and letter to an editor of any news paper
and letter of complaining

and topic for informal letters are 
letter to your friend
letter to ur relatives 
letter to ur cousin who lives in hostel
and letter to ur grandfather etc.
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Formal--write a letter to ur principle for delay in fee submition
letter to traffic headquatter complaing about reckless driving
informal----------letter to young bro saying the imp of news paper
letter to frnd inviting to ur b'day party.
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