Calculate

A simple mental calculation trainer with this features:

- contains the four elementary arithmetic operations and the prime factorization
- configurable task types and diffculty
- configurable number input field
- statistic about the performance
- multi user support
- time trial mode (solve as many tasks as possible within a defined time) with tasks depending on the wall clock time so that one can play "against" another one with the same tasks

- monochrome icon
- updated buildtools and dependencies