概率题,请大家帮忙

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1. An inspector working for a manufacturing company has a 99% chance of correctly identifying defective items and a 0.5% chance of incorrectly classfying a good item as defective. The company has evidence that its line produces 0.9% of nonconforming items.
a)What is the probability that an item selected for inspection is classfied as defective?
b)If an item selected at random is classfied as nondefective, what is the probability that it is indeed good.

2.Transaction as a computer database are either new items or changes to previous items. The addition of an item can be completed less than 100 millseconds 90% of the time, but only 20% of changes to a previous item can be completed in less than the time. If 30% of transactions are changes, what is the probability that a transaction can be completed in less than 100 millseconds?
 
1,a)0.991*0.005+0.009*0.99
b)(0.991*0.005)/a)
2,0.5*0.9*0.3+0.2*0.5*0.3

应该是的
 
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