John started driving on a highway at a constant speed of R miles per hour at 13:00. Then, 2 hours later, Tom started driving on the same highway at a constant speed of 5R/4 miles per hour at 15:00. If both drivers maintained their speed, how many miles had John driven on the highway when Tom caught up with John? Answers.
A) 6R  B) 8R  C) 10R  D) 12R  E) 14R
In 2 hours, John drove 2R miles.   Tom can catch John at a rate of 5R/4 - R = R/4 miles per hour.   The time it takes Tom to catch John is 2R / (R/4) = 8 hours.   In 8 hours, John can drive another 8R miles for a total of 10R miles.   The answer is 10R.
<result>C</result>
John started driving on a highway at a constant speed of R miles per hour at 13:00. Then, 2 hours later, Tom started driving on the same highway at a constant speed of 5R/4 miles per hour at 15:00. If both drivers maintained their speed, how many miles had John driven on the highway when Tom caught up with John?