This app I made for me. Like its sister app, Exploring Music: Music Notes Flash Cards, this app did sort of originate from the Burger King sessions with my father, asking about the sharps and flats and which key had how many sharps, etc. The music notes had a practical application for me at the time, I was taking piano lessons and so had no real choice but to learn them in even the barest capacity just to get by. The key signatures I left behind. Now, as an adult, I think back on the red index cards we had around the house with the sharps, flats, and which major and minor keys were which. To refresh my own memory, and to help me learn, I made this app. I hope anyone else in the same situation might find them useful.
Good luck.