We know that each month, he gets 4 coins to add to his collection so that would represent 4m. And we know that he started with 20. The last thing that we know is that we will need a greater than symbol to represent more than 50. Therefore the equation is 20 + 4m > 50.
(if he gathers 4 coins each month you will need the variable by it. he was given 20 coins so it is added on to how many coins he has. plus it would be greater than because the question asked when he will have MORE than 50 coins.)
