A task is assigned to a group of 18 people consisting of 6 male adults, 6 female adults and 6 children, each person having a different rate of working. Everyday a subgroup of 3 persons consisting of 1 male adult, 1 female adult and 1 child work together, such that no subgroup with the same combination works for more than one day. In this way, the entire work is completed in exactly 216 days. Find what fraction of the total work is completed, if all the 18 people work together for a single day.