Final answer:
The tanker truck has been filling the reservoir for approximately 2.917 minutes.
Step-by-step explanation:
To calculate the time it took for the tanker truck to fill the gas station's reservoir, we can use the formula:
Time = Quantity / Rate
In this case, the quantity is 35 gallons (since the reservoir is now 35 gallons full) and the rate is 12 gallons per minute. Plugging these values into the formula, we get:
Time = 35 gallons / 12 gallons per minute = 2.917 minutes
Therefore, the tanker truck has been filling the reservoir for approximately 2.917 minutes.