74. A school uses a drug fumigation method to disinfect classrooms. It is known that when the drug is burning, the content of the drug $y(\mathrm{mg})$ per cubic meter of air is directly proportional to the time $x$ (min); after the drug burns out, $y$ is inversely proportional to $x$ (as shown in the figure). It is measured that the drug burns out in $8 \mathrm{~min}$, at which point the content of the drug in the air per cubic meter is $6 \mathrm{mg}$. Please answer the following questions based on the information provided in the question:
(1) When the drug is burning, the function relationship of $y$ with respect to $x$ is: $\qquad$, the range of the independent variable $x$ is: $\qquad$, after the drug burns out, the function relationship of $y$ with respect to $x$ is: $\qquad$;
(2) Research indicates that students can return to the classroom only when the content of the drug in the air per cubic meter is below $1.6 \mathrm{mg}$. How many minutes at least from the start of disinfection must pass before students can return to the classroom? $\qquad$ $\min$;
(3) Research indicates that the air-borne pathogens can be effectively killed only when the content of the drug in the air per cubic meter is no less than $3 \mathrm{mg}$ and lasts for no less than $10 \mathrm{~min}$. Is this disinfection effective? Answer: $\qquad$
(1) $y=\frac{3}{4} x,(0 \leq x \leq 8) ; \quad y=\frac{48}{x}$
(2) 30
(3) valid