At a construction site, pulley system X raises a 20 kg bucket of mortar through a vertical height of 6.0 m in 12 s. Pulley system Y raises a 30 kg bucket through a vertical height of 4.0 m in 8.0 s. What is the correct comparison of their mean useful power outputs?
The useful power of Y is greater.
The useful power of X is greater.
The useful powers cannot be compared from these data.
The useful powers of X and Y are equal.