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 negative, B=not being a marijuana user, then $P(A\cup B)=\frac{157}{300}+\frac{178}{300}-\frac{154}{300}=\frac{181}{300}=0.603.$