Added exercise for sets intuition

This is an interactive exercise for getting intuition about sets. It should be at the beginning of the section "Basic Set Operations" (which is in Math > Probability and Statistics > Independent and Dependent Events, although I think this section could be earlier since these sets basics are really easy to understand and useful for all other math.) The first video in this section is https://www.khanacademy.org/math/probability/independent-dependent-probability/basic_set_operations/v/intersection-and-union-of-sets, but I think the exercise could be even before the first video.
I will be grateful for any feedback, advice or error notifications. And (again) I would like to ask for corrections of my English in the exercise since I am not a native English speaker and I am not good at English so I guess there are some grammatical mistakes.