Ok so basically, Beth is traveling at 8MPH. And her bro leaves 1/4 of an hour later, this means she has cycled 8/4 = 2 miles. After another 15 minutes, Beth will have cycling 4 miles, while Brandon will have carted 12/4 = 3 miles. Repeat this, and Beth has cycled 6 miles in 45 minutes, and Brandon has also gone 6 miles in 30 minutes.