Recently I have got a question as below:
Write a letter to one of your former colleagues about your upcoming visit to the city where you both worked together. In this letter please include:
-Your current life condition -Detailed plan of your visit -Meeting arrangments with him
How should I approach this letter? in formal or informal way? Because I am writing a letter to one out of at least a hundred colleagues. That must make him one of my close ones. So, in my opinion, informal tone seemed appropriate.