Occasionally it's possible that the local app cache data may get into a corrupt state which could cause the app to crash immediately when launching.
On android please uninstall the app (don't worry, your data is afe!), go to the Play store, and install the app again.
On iOS sometimes iCloud prevents the cache from being cleared even after uninstalling and reinstalling the app.
In these cases you will need to follow these steps (don't worry your data is safe!):
- Force close the Pool Math app
- Navigate to the Settings app
- Scroll down to and select "Pool Math"
- Turn on "Reset App Cache"
- Launch the Pool Math app and sign in again.