The power output of pushing a 15.0 kg box across the floor at a steady speed of 1.25 m/s for 16.0 s, with a coefficient of kinetic friction of 0.680, can be calculated using the formula:
Power = Force x Velocity
Force = mass x acceleration due to friction
Acceleration due to friction = coefficient of kinetic friction x gravitational force
Power = (15.0 kg x 0.680 x 9.8 m/s2) x (1.25 m/s)
Power = 128.4 Watts
Therefore, the power output of pushing the box for 16.0 s is 128.4 Watts.