Let me try to give you a couple hints and see if you can try to tackle this on your own. If you're still having trouble, come back and let me know.
First of all, remember that you can always use a reference to a child when a reference to a parent is expected. Let's say, for example, you have two classes, Animal and Cat (Cat extends Animal). Anywhere that you would expect to find a reference to an Animal object, you are free to use a Cat object because, after all, a Cat "IS A" Animal. Going the other way does not work. If you're expecting a reference to a Cat object, you can't use a reference to an Animal object because an Animal is not a Cat. So, always keep that rule handy - you can use a child anywhere a parent is expected.
Secondly, downcasting will fail if the run-time type of the object being cast isn't a type of the object you're casting to. So, for example, if you have an Animal object and you try to cast that to a Cat object, you're going to get a ClassCastException, because an Animal is not a Cat. However, if you try to cast a Cat object to an Animal, you're just fine because a Cat is a Animal.
Keep those rules in mind and try to figure out what's happening in each case. Let me know what you come up with.