Pandas data manipulation

Hello, I have a Python pandas project i am working on and i am stuck on something probably ridiculously simple. I have a list of arcade properties and i have the TWPD(Theo Win Per Day of each game in each arcade. I have another column identifying each game as Core or NonCore. I want to get the Average TWPD of each Arcades Core product only. Then i want to put it in a column named "ARC_TWPD_AVG". This will put the TWPD Core average next to each game, Core and Noncore based by Arcade. I have attached a simple python code and the input file and expected output file. I manually manipulated the "ARC_TWPD_AVG" column so you can check your work. This is probably 1 or 2 lines of code. I was able to get it to drop the NONCore from the calculation, but it dropped it from the final dataframe. I could not figure out how to do it properly.