How to do it...

1. Add the getWeightKg(int pounds) method implementation:

     public interface Truck extends Vehicle {
         int getPayloadPounds();
         default int getPayloadKg(){
            return (int) Math.round(0.454 * getPayloadPounds());
         }
         static int convertKgToPounds(int kilograms){
            return (int) Math.round(2.205 * kilograms);
         }
         default int getWeightKg(int pounds){
            return (int) Math.round(0.454 * pounds);
         }
     }

2. Remove the redundant code by using the private interface method:

    public interface Truck extends Vehicle {
        int getPayloadPounds();
        default int getPayloadKg(int pounds){
            return convertPoundsToKg(pounds);
        }
        static int convertKgToPounds(int kilograms){
            return (int) Math.round(2.205 * kilograms);
        }
        default int getWeightKg(int pounds){
            return convertPoundsToKg(pounds);
        }
        private int convertPoundsToKg(int pounds){
            return (int) Math.round(0.454 * pounds);
        }
    }