Here is the data

10 mg sub A Placebo
headache 15 65
n headache 17 3

Question
If two different subjects are randomly selected, find the probability that they will both have a headache.

I think that it would be 15/100*17/99 but the answer that this gives is not right.:(:confused:

any help in the right direction would be grealty appreciated

I don't understand your data, could you please clarify so I can help you?

Yes. The headache row is 15 had headache with 10 mg sub a and 65 had headache with placebo. Second row is 17 had no headache with 10 mg of sub a and 3 with placebo

Okay, you need to take the total with headaches, over the total number of patients, twice where the second one is minus one person with a headache so it's

P = \frac{15+65}{100}*\frac{15+65-1}{100-1}

Does that make sense?
Does it give the right answer?