Hello Piet,
Please consider our Homework policy FAQ: Homework Policy
In light of this, I wont directly answer your query, but ask you some questions to get you along.
What part(s) are you stuggling with?.
Can you create a variable that shows anything at all ?
can you check whether a variable meets a single condition?
Whats the closest you believe that you came towards answering the question for yourself... what did you try ?