Answer:
539.7 meters above sea level.
Explanation:
Okay in order to solve this problem we can take it one step at a time. First, he starts at an altitude of 120.4 meters above sea level.
Then he hikes up an additional (meaning we add) 850.0 meters.
120.4 + 850.0 = 970.4
Next, Chandler hikes down (meaning we subtract) 430.7 meters.
970.4 - 430.7 = 539.7
Therefore, we can see that when Chandler is in the valley he is at an altitude of 539.7 meters above sea level.