Final answer:
The time [t] when the volume of water in the tub is less than or equal to 640 gallons is t ≥ 8 minutes. Therefore, the time [t] when the volume of water in the tub is less than or equal to 640 gallons is t ≥ 8 minutes.
Step-by-step explanation:
To find the time [t] when the volume of water in the tub is less than or equal to 640 gallons, we can use the equation:
Volume of water in tub = Initial volume - Drainage rate x Time
Since the initial volume is 800 gallons and the drainage rate is 20 gallons per minute, the inequality can be written as:
800 - 20t ≤ 640
Simplifying the inequality, we have:
-20t ≤ -160
Dividing both sides by -20, we get:
t ≥ 8
Therefore, the time [t] when the volume of water in the tub is less than or equal to 640 gallons is t ≥ 8 minutes.