The mesosphere is the layer of Earth’s atmosphere between 50 kilometers and 85 kilometers above Earth’s surface. At a distance of 50 kilometers from Earth’s surface, the temperature in the mesosphere is `-5^(@)` Celsius, and at a distance of 80 kilometers from Earth’s surface, the temperature in the mesosphere is `-80^(@)` Celsius. For every additional 10 kilometers from Earth’s surface, the temperature in the mesosphere decreases by `k^(@)` Celsius, where k is a constant. What is the value of k ?