Thanks for the comments, especially for clemens' bug report!
As the deadline already passed, I can't change things anymore
Maybe a bugfix release after some time?

the exercise towers...
I had no time to explain it well enough
they look like:
5
+
5
=
27

with picking up numbers with e,
you can change the numbers on the boxes (also with e)
so ...27 should be 10

indeed a config menu would be nice,
I also had no time for that, as I gave up on the project for about a week.