I’m hoping this is OK to post.
This is my favourite.
Ankidroid is a study app that uses a combination of flash cards and timed interval learning.
The questions that are difficult will pop up with greater frequency.
The easier questions will extend to the point they are hardly seen.
Highly recommended for older brains that resist learning and require something more “forceful” to get the information to stay in there 
Anyone else use this or study apps like it?
All recommendations appreciated.