This app was mentioned in 1 comment, with an average of 1.00 upvote
On the other hand, you could brush up on your mental maths :) This one's mine but there are others, some are better I have to admit, but I'm working on upgrades. Search mental maths or vedic maths on the play store.
https://play.google.com/store/apps/details?id=appinventor.ai_adrianpeirson60.Vedic11