In: Statistics and Probability
Before each class, I either drink a cup of coffee, a cup of tea,
or a cup of water. The probability of coffee is 0.7, the
probability of tea is 0.2, and the probability of water is 0.1. If
I drink coffee, the probability that the lecture ends early is 0.3.
If I drink tea, the probability that the lecture ends early is 0.2.
If I drink water, the lecture never ends early.
1) What’s the probability that I drink tea and finish the lecture
early?
2) What’s the probability that I finish the lecture early?
3) Given the lecture finishes early, what’s the probability I drank
coffee?
The probability of coffee, P[ Coffee ] = 0.7
The probability of tea, P[ Tea ] = 0.2
The probability of water, P[ Water ] = 0.1
P[ I drink coffee, the lecture ends early ] = P[ lecture ends early | coffee ] = 0.3
P[ I drink tea, the lecture ends early ] = P[ lecture ends early | tea ] = 0.2
P[ I drink water, the lecture ends early ] = P[ lecture ends early | water ] = 0
1) What’s the probability that I drink tea and finish the lecture early?
P[ I drink tea and finish the lecture early ] = P[ lecture ends early | tea ]*P[ Tea ]
P[ I drink tea and finish the lecture early ] = 0.2*0.2
P[ I drink tea and finish the lecture early ] = 0.04
2) What’s the probability that I finish the lecture early?
P[ I finish the lecture early ] = P[ I drink coffee and finish the lecture early ] + P[ I drink tea and finish the lecture early ] + P[ I drink water and finish the lecture early ]
P[ I drink coffee and finish the lecture early ] = P[ lecture ends early | coffee ]*P[ Coffee ]
P[ I drink coffee and finish the lecture early ] = 0.7*0.3
P[ I drink coffee and finish the lecture early ] = 0.21
P[ I drink tea and finish the lecture early ] = P[ lecture ends early | tea ]*P[ Tea ]
P[ I drink tea and finish the lecture early ] = 0.2*0.2
P[ I drink tea and finish the lecture early ] = 0.04
P[ I drink water and finish the lecture early ] = P[ lecture ends early | water ]*P[ Water ]
P[ I drink water and finish the lecture early ] = 0.1*0
P[ I drink water and finish the lecture early ] = 0
P[ I finish the lecture early ] = 0.21 +0.04 + 0
P[ I finish the lecture early ] = 0.25
3) Given the lecture finishes early, what’s the probability I drank coffee?
P[ I drank coffee | the lecture finishes early ] = P[ I drink coffee and finish the lecture early ]/P[ I finish the lecture early ]
P[ I drank coffee | the lecture finishes early ] = 0.21/0. 25
P[ I drank coffee | the lecture finishes early ] = 0.84