# Probability Help!

• Apr 29th 2009, 08:04 PM
rafaeli
Probability Help!
Suppose that it is known that in an entrance exam, 10% cheat on the exam, and 90% do not. About 70% of the cheaters get a perfect score, while 20% of the noncheaters get a perfect score.

1) What is the probability that a randomly selected student taking the exam will get a perfect score?
2) Give that a student has a perfect score on the exam, what is the conditional probability that it was obtained by cheating?
• Apr 29th 2009, 08:24 PM
mr fantastic
Quote:

Originally Posted by rafaeli
Suppose that it is known that in an entrance exam, 10% cheat on the exam, and 90% do not. About 70% of the cheaters get a perfect score, while 20% of the noncheaters get a perfect score.

1) What is the probability that a randomly selected student taking the exam will get a perfect score?
2) Give that a student has a perfect score on the exam, what is the conditional probability that it was obtained by cheating?

Try drawing a tree diagram to see that:

1) (0.1)(0.7) + (0.9)(0.2) = ....

2) (0.1)(0.7)/[(0.1)(0.7) + (0.9)(0.2)] = ....