#### Answer

a)0.905
b)0.993
c)They are not equal.

#### Work Step by Step

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.