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?