Find the speed of an aircraft that is 0.8 km on the way. braked for 20 s.

Let the plane at the beginning of deceleration have a speed V and move with an acceleration a.
The origin of coordinates will be located at the point of the beginning of braking and will direct the x-axis in the direction of movement.
The x coordinate of the aircraft at time t:
x = Vt + at² / 2 (1).
Let us express the acceleration in terms of the initial V and final speed Vк = 0:
a = (Vk – V) / t = (0 – V) / t = -V / t.
Substitute the found expression for a into (1):
x = Vt + (-V / t) * (t² / 2);
x = Vt – Vt / 2 = Vt / 2;
V = 2x / t = 2 * 800 m / 20 s = 80 m / s;
Answer: 80 m / s.