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 7 miles per day and it will take them two days, how many miles long is the route down the mountain?
Select the correct option:
A) 18 B) 19 C) 20 D) 21 E) 22

Rationale: "on the way down , the rate is 1.5 * 7 = 10.5 miles per day . the distance of the route down the mountain is 2 * 10.5 = 21 miles . the answer is d ."
Answer: D