Hi
I didn't understand the highlighted part. Cud u pls elaborate?
Thanks
Nidzo
We are told that 5% of bags contain a forbidden item, therefore 95% do not.
Given the phrasing of the question, one can conclude that there are four possible outcomes which can occur:
1) A banned item triggers the alarm (Let that be represented by BT)
2) A banned item that does not trigger the alarm (Let that be represented by BnT)
3) A non-banned item which triggers the alarm (Let that be represented by nBT)
4) A non-banned item which does not trigger the alarm (Let that be represented by nBnT)
The probability that a triggered alarm is a banned item will be (pBT)/(pBT + pnBT) [where p represents probability]
(1) If a bag contains a forbidden item, there is a 98% chance that it triggers the alarm.
From this statement one can work out the pBT. However, without pnBt one cannot solve this question.
INSUFFICIENT
(2) If a bag doesn't contain a forbidden item, there is an 8% chance that it triggers the alarm.
From this statement one can work out the pnBT. However, without pBt one cannot find the probability that a randomly chosen bag that triggers the alarm will contain a banned item.
INSUFFICIENT
(1+2)
Putting the statements together will yield values for both pBT and for pnBT and thus one will be able to fill in the equation (pBT)/(pBT + pnBT) and have the probability that a randomly chosen bag that triggers the alarm will contain a banned item.
SUFFICIENT
ANSWER C
THE MATH:
(1) If a bag contains a forbidden item, there is a 98% chance that it triggers the alarm.
\(\frac{5}{100}*\frac{98}{100}\)
\(\frac{1}{20}*\frac{49}{50} = \frac{49}{1000}\) which is BT.
(2) If a bag doesn't contain a forbidden item, there is an 8% chance that it triggers the alarm.
\(\frac{8}{100}*\frac{95}{100}\)
\(\frac{2}{25}*\frac{19}{20} = \frac{19}{250}*\frac{4}{4} = \frac{76}{1000}\) IS nBT
(1+2)
Using the above values for BT and nBT in the equation: (pBT)/(pBT + pnBT)
\(\frac{\frac{49}{1000}}{\frac{49}{1000}+\frac{76}{1000}}\)
\(\frac{\frac{49}{1000}}{\frac{125}{1000}}\)
\(\frac{49}{1000}*\frac{1000}{125}\)
\(\frac{49}{125}\) [multiply through by \(\frac{8}{8}\)]
\(\frac{392}{1000}\)