Hey everyone I'm Bratin and I'm a class 10 student hailing from Kolkata. I've been into math contests for a while now and enjoy combinatorics and geometry the most. In this post I'll dive into the topic of double counting by working through some problems and hopefully be able to showcase how to employ the technique when the need arises for the same. Put simply, double counting is basically counting in two different ways (duh). The main goal is to count a single quantity in $2$ different ways. There really is no theoretical concepts attached to it. It's just basically elementary combinatorics with ingenuity since you need to figure out what are the $2$ "ways" in which you can consider counting. Below are few problems to get you started. I've discussed the solutions to all of the problems mentioned, with motivation/comments. It is however advisable that the readers try their hand independently at them before moving on to read the solutions. IMO 1987...