



 
Hi Rougi. a) Rework the question backwards. What is the probability he doesn't forget his umbrella anywhere? It is 0,9 x 0,7 x 0,4. The answer to the question is the complement of this. b) He forgot it somewhere, so the relative probabilities are what is important in this question. In this case it is relative to his forgetting his umbrella so you want to form a fraction where the denominator is the probability you calculated in part a). The numerator is the probability that when he forgot his umbrella he forgot it in the taxi. To do so he would need to not forget his umbrella on the bus AND not forget his umbrella in the bar AND forget his umbrella in the taxi. Hence the numerator is 0,9 x 0,7 x 0,6. Cheers,  


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