SOLUTION: A young boy is stupidly entering a store. He has a 1/4 probability in buying a drink. 1/5 probability of buying an ice cream bar. There's a 1/10 chance of him buying both. If it is

Algebra ->  Probability-and-statistics -> SOLUTION: A young boy is stupidly entering a store. He has a 1/4 probability in buying a drink. 1/5 probability of buying an ice cream bar. There's a 1/10 chance of him buying both. If it is      Log On


   

Question 976042: A young boy is stupidly entering a store. He has a 1/4 probability in buying a drink. 1/5 probability of buying an ice cream bar. There's a 1/10 chance of him buying both. If it is known that he bought at least one of these items, find the probability that he bought both of them.
Answer by FrankM(1040) About Me  (Show Source):
You can put this solution on YOUR website!
If we think of this as a grid 4x5, there are 20 spaces.
5/20 he gets a drink
4/20 he gets ice cream
2/20, both.
So there are really just 7 outcomes that have a purchase. 3 drink only, 2 both, 2 ice cream only.
If he bought at least 1, he is in the space of these 7, and has a 2/7 chance he bought both.