Probability question?

Urn A has 2 red and 3 blue marbles, and urn B has 4 red and 1 blue marble. Peter selects an urn by tossing a coin, and takes a marble from that urn.

a)Determine the probability it’s red.
b)Given that the marble is red, what’s the probability that it came from B?

I’m having trouble how to illustrate this problem. Should I use a tree diagram?

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