Answer:
<em>The speed of the plane after it decelerates is 50 m/s</em>
Explanation:
<u>Motion with Constant Acceleration</u>
When an object gains or losses velocity in time, it acquires acceleration. If this value is constant, we can calculate the final velocity (or speed in scalar terms) as:

Where vf is the final speed, vo is the initial speed, a is the constant acceleration, and t is the time the acceleration is acting.
The plane is originally traveling at vo=80 m/s and it slows down at a constant rate of
during t=120 seconds. Note we have added the negative sign to the acceleration because the plane is slowing down, i.e., the acceleration is against the speed.
Thus, the final speed is:



The speed of the plane after it decelerates is 50 m/s