## Elementary Statistics (12th Edition)

a)We know that $probability=\frac{number \ of \ good \ outcomes}{number\ of\ all\ outcomes}$, hence $P(Negative|No \ drugs)=\frac{860}{860+90}=0.905.$ b)We know that $probability=\frac{number \ of \ good \ outcomes}{number\ of\ all\ outcomes}$, hence $P(No \ drugs|Negative)=\frac{860}{860+6}=0.993.$ c)$0.905\ne 0.993$, they are not equal.