At this point of the Earth's surface, the Stratosphere layer is at most at 50 km, and at least at 16 km altitude.
If y represents a height which is in the Stratosphere layer, then we know that y is at least 16 km, which is represented by

,
and y is at most 50 km, which is represented by

.
Combining these 2 inequalities we have:

, where y is in km