If she's going up the mountain, she's going X kilometers up the mountain at 2 kilometers per hour. Remember if you are going to drive 100 miles at fifty miles an hour, you just divide the 50 into the 100. So, her distance here is X, divide the speed into it. So, you have X/2. Now she's coming down the same distance she went up, isn't she? So you have X/3 as the time it took her coming down. So your equation you set up is X/2 + X/3=10. Well your least common denominator here is going to be 6, so you have (2X+3X)/6=10. Therefore, multiplying each side by six, gives you 5X=60; the distance is X=kilometers, and that answer is at C.