there are two roads from A to B, and two roads from B to C.each of the four roads has independently probability 1/10 of being blocked by snow.What is the probability that one can drive from A to C?:wave::wave::wave:

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I have tried to solve it by tree diagram, and suggest that a union operations between the four roads. but unfortunately the resulted probabilities were larger than 1!!, can't any one give me a hint please?

If road 1 and road 2 lead from A to B, we need P(1 or 2). Similarly, if road 3 and road 4 lead from B to C we need P(3 or 4). Then, how do we use this info to get from A to C? (P(1 or 2) is the probably that one road or the other will take us there)

You're welcome. There are a few approaches to this problem. One of them makes use of the complement rule. The probability that at least one road is open is... Try to figure it out this way and see if your two answers match.