A packing crate with mass 80 kg is at rest on a smooth floor. At t= 0 a horizontal force is applied to the crate, The force is a constant 80 N for 12 seconds, and then decreases linearly with time so that it becomes zero after an additional 6 seconds. What is the final speed of the crate, 18 seconds after the force was first applied? ( 15 m/s)