Keiran, whose mass is 45 kg, is riding his 5.0 kg skateboard down a level, horizontal sidewalk with a constant speed of 6.0 m/s when he rolls across a 5.0-m-long patch of sand on the pavement. The sandy pavement provides a force of friction of 6.0 N. How much work is done by the patch of sandy pavement on the skateboarder? (As with this and all work-out questions, remember to work symbolically first to be eligible for full credit.)