Chrystal’s vehicle speed is 30 miles per hour. Ascending the mountain decreases its speed by fifty percent, and descending the mountain increases its speed by twenty percent. If the distance going to the top of the mountain is 60 miles and the distance going down to the foot of the mountain is 72 miles, how many hours will Crystal have to pass the whole mountain? Let's think first. Chain of thought:
The vehicle's speed decreases to 30 x 0.50 = 15 miles per hour when ascending to the top of the mountain. So, the total time Crystal will have to spend going to the top of the mountain is 60 / 15 = 4 hours. And the speed of the vehicle increases by 30 x 0.20 = 6 miles per hour when going down to the foot of the mountain. So, the total speed of the vehicle when going down is 30 + 6 = 36 miles per hour. Thus, Chrystal will have to spend 72 / 36 = 2 hours to descend from the mountain. Therefore, the total hours she will spend to pass the whole mountain is 4 + 2 = 6 hours.
Therefore, the answer is 6.