Answer:
First, we need to find how far ahead Marshall was. Since he had been biking at 20 mph for one hour, he had gone 20 miles.
Next, we need to find how long it will take Brett to catch up to Marshall. In order to do this, we need to find how much faster Brett is going than Marshall. We do this by subtracting Marshall's speed from Brett's speed.
60 - 20 = 40. So, Brett is catching up to Marshall at 40 mph. Now, we figure out how long it will take for someone going 40 miles per hour to go 20 miles. We find this by dividing 40 miles per hour by 20. This is equal to 1/2 hour. So, it will take Brett 0.5 hours to catch up to Marshall. This is the same as A, so A is the correct answer.
We can check our answer by seeing how far Marshall and Brett will have gone. Marshall will have been biking for 1.5 hours, so we multiply 20 * 1.5 = 30. Marshall went 30 miles.
Brett drove for .5 hours at 60 mph, so he went 30 miles. Since Brett and Marshall went the same distance, our answer is correct.
