A group of hikers is planning a trip that will take them up a mountain using one route and back down using another route. They plan to travel down the mountain at a rate of one and a half times the rate they will use on the way up, but the time each route will take is the same. If they will go up the mountain at a rate of 6 miles per day and it will take them two days, how many miles long is the route down the mountain?  Select:
A) 16  B) 17  C) 18  D) 19  E) 20

Rationale: "on the way down , the rate is 1.5 * 6 = 9 miles per day . the distance of the route down the mountain is 2 * 9 = 18 miles . the answer is c ."
Answer: C