You can use the fact that both the irrationals and rationals are dense in the reals.
When working in the reals, a ball is just an interval - I assume you can use intervals?
How would I show that the set of rational numbers Q is neither closed nor open? Like which theorems would I use? I have been trying this for hours now and can't seem to get anywhere. thanks for your help
ps- we havent learned the definition of a ball, so please stick to the other more basic definitions