Add more help for most difficult exercises

[asimell]

The slider exercise uses a lot of time explaining how to get the magical mathematical formula to get the for loop to work. The main idea of the exercise is to learn the proper syntax and usage of the for-loop, not to learn how to do maths, so the formula could be given for the participants and they would just need to implement the for loop using the formula.

There's probably other exercises as well, which have similar unnecessarily complex tasks for the exercise purpose.