Click here for an Excell sheet you can use to create an infinite amount of new problems (click at the bottom to change from divisions to multiplications to additions and use F9 to generate new problems). Click here instead for the same sheet but with the answers this time (you can practice with that if you first hide the answers with a piece of paper or something else).
Click here for a site you can use to practice the optional calendar calculation challenges (make sure to choose the right options).
Other apps and games that have been recommended (in no particular order):
- https://play.google.com/store/apps/details?id=volodymyr.kuprych.mentalarithmetictrainer&hl=en_US (for Android)