Pulling Weeds

Dr. Hong is taking Leo, Angela, and Elon to pull the weeds out of their front yard. If Dr. Hong does all the work on his own, it will take him 10 hours. If Leo and Angela work together, it will take them 15 hours. If Leo and Elon work together, it will take them 20 hours. If Angela and Elon work together, it will take them 30 hours. How many hours does it take if all four of them work together? Please keep two digits after the decimal point for the final answer. (We can assume that the weeds don’t grow when they work on the lawn.)