In symbols, you were asked to calculate P(NA|ND).

Question
Is this one of the probability items on the tree?

Answer
No. We know this because a branch corresponding to P(NA|ND) would go from ND to NA:

But there is no such branch on the tree.

Question
So how do we calculate it?

Answer
We go back to the formula for conditional probability

P(NA|ND)

=

P(NAND) P(ND)

Here is the tree diagram with both these probabilities shown:

	Stage 1	Stage 2	Outcome	Probability
			FDA approves & stock price does not double	0.04
			FDA does not approve & stock price does not double	0.18	P(NAND)
			P(ND):	0.22

Therefore,

 

P(NA|ND)	=	P(NAND) P(ND)
	=	0.18 0.22	=	9 11

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