probability(statistic)

In one canteen, they sell sandwich and coffee. A group of 1000 buyer, 400 bought sandwiches, 300 purchased coffee, and 200 purchased sandwich and coffee. If the buyer chosen at random bought sandwiches coffee, what is the probability they also bought coffee?

So is it 200+300/1000= 0.5

• It's best that you show us your work. Where did you get stuck? – Student May 10 at 12:45
• so there's 1000 buyer, 400 bought sandwiches, 300 coffee and 200 purchased sandwich and coffee and the left 100 buyers? – sls May 10 at 12:52
• Probably not everyone has to buy something. You have a conditional probability, which formula do you use to compute such probabilities? How many people only buy a sandwich? Also, update your work into your question (also do this in the future :) ), instead of in the comments. – Student May 10 at 13:07

Let $$A$$ be the event of buying a sandwich and $$B$$ be the event of buying coffee. We find that $$|A| = 400, \quad |B| = 300 \quad \text{ and} \quad |A \cap B| = 200$$
The probability of buying a sandwich equals $$P(A) = 400/1000 = 0.4$$.
You are tasked to compute the probability $$P(B | A)$$, which is a conditional probability. Use the formule for conditional probability and the answer to the previous question to compute the conditional probability.