If a car comes down the road every 20 minutes, then it takes 20 minutes for one car to travel the entire 3-mile road.
So, it takes 20 minutes for one car to travel 3 miles, which is equivalent to 20/60 = 1/3 of an hour.
If 30 cars have come down the road, then it has taken 30 * (1/3) = 10 hours.
Therefore, 10 hours have passed once 30 cars have come down the road.
#### 10
The answer is: 10
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
There is a road stretching 3 miles from the base camp to the chain challenge. Every 20 minutes, a car comes down the road from the base camp towards the chain challenge. How many hours have passed once 30 cars have come down the road?

### Answer:
