on p.112 on Fadem's HY Behavioral Science (2nd edition):

20% of patients who take a new drug for hypertension develop nausea. If two patients (patients A and B) take the drug, calculate the probability that at least one patient (i.e. either A or B or both A and B) will experience nausea.

The explanation says that you should add the probability of patient A experiencing nausea to the probability of B experiencing nausea and then subtract the probability of both A and B experiencing nauseas, so

0.20 + 0.20 -0.04 = 0.36

(0.04 is the probability that both A and B will experience nausea).

My question is, why do you subtract 0.04? Don't you add it to 0.2 and 0.2? For the life of me, I can't figure this one out. Thanks for your help!!!