To solve this, you must look to see how many miles per hour the group is travelling. This can be found by multiplying 15 by 3, because 20*3 = 60. You will find that it is 45 mph. Then, divide that by 2, to get 22.5. That is how far they will go in a half an hour. Then, multiply 45 by 2, and add 22.5 to that. The answer you find is then 112.5 miles.
