Explanation

The constant speed of this car can be written as . Since this rate doesn't change in the question, we will solve by setting this equal to an equivalent fraction:



The question is what goes in the place of the question marks. Since the numerator is minutes, that will be 40 minutes, the time from 3:00 pm to 3:40 pm. To determine the distance, we could draw a little diagram:



This diagram is a schematic and not exact, in the sense that the car is actually going straight along the line and then turning around and going right back along the straight line, not along a curve. The curve is how we can sketch it to understand going x miles, turning around, and going x miles back. The total distance is x + x + 30 miles. Plugging in, we have



Cross-multiplying gives





So, x=10. Let's confirm this is what we are looking for, checking the sketch we made. We've found the distance the car can go before it has to turn around. That's right.

The correct answer is (A).