Elementary Statistics (12th Edition)

We know that $probability=\frac{number \ of \ good \ outcomes}{number\ of\ all\ outcomes}$, and also $P(A\cup B)=P(A)+P(B)-P(A\cap B)$. Let A=being tested positive, B=being a marijuana user, then $P(A\cup B)=\frac{143}{300}+\frac{122}{300}-\frac{119}{300}=\frac{146}{300}=0.487.$