probability – Fight with the Bayes network

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I am in an automatic learning course and the Bayes networks were presented in such an abstract way that I find it very difficult to understand how to use it. And all the examples that I can find, the final numbers seem to appear magically without any explanation. Please help!

I am already stuck in problem 1. I think part of the idea is that $ G $ will not be part of the simplified expression because it is not in the way of $ B $ to $ W $ On the graph (DAG).

I want to use the Bayes formula:

P (B = tr | W = tr) = dfrac {P (B = tr, W = tr)} {P (W = tr)}

But I do not know how to continue from there, or expand this expression rather.

I guess if I understand 1, 2 it will be easier. Either way, I'm happy with any help I get. Thank you