Seventy percent of the kids who visit a doctor have high temperature and 30% of the kids who have high temperature, also have smallpox. What's the probability that a kid who goes to the doctor's has a temperature and smallpox?
supose you have 100 kids, 70 of them(100*0.7) have high temprature. Now, 70 is your sample, 30% of them have smallpox, so 70*0.3=21. So your probability of taking kid with both simptoms is equal to 21/100=70/100*21/70=0.7*0.3