The average acceleration while the thrusters are on is 80 m/s^2.
Let:
a = acceleration = ? m/s^2
vi = initial velocity = 20,000 m/s
vf = final velocity = 24,000 m/s
t = time when shuttles were on = 50 seconds
a = (vf - vi) / t
a = (24,000 - 20,000) / 50
a = (4,000) / 50
a = 80 m/s^2
