#### Answer

a)300
b)178
c)0.593

#### Work Step by Step

a) As can be seen in the picture, there are 300 subjects in total.
b) As can be seen in the picture, there are 178 subjects who did't use marijuana.
c)We know that $probability=\frac{number \ of \ good \ outcomes}{number\ of\ all\ outcomes}$, hence the probability of selecting a subject who didn't use marijuana:$\frac{178}{300}=0.593.$