First we take Sean to be equal to x. The question states that Farhan has four times as many as Sean and he therefore is 4x. When Farhan loses 180, Sean gains it, and then they are the same. Hence the equation. Solving it for x gives how many Sean had at the start.

There has been no total loss/gain between these two out of the system. That is, no stamps have been lost one of them that hasn't been exactly gained by the other . Therefore. the original number of stamps they had before the exchange will be the same whether or not we consider the exchange. Therefore, for simplicity, I added x and 4x (their original numbers) and solved for the total number of stamps.